Post on 08-Aug-2020
Functional analysis Distant Learning. Week 3.
Functional analysis
Lesson 9.
April 21, 2020
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . }
−→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→
{ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
Review
In the WEIGHTED L2 SPACE we applied G-S orthogonalization:
{1, x , . . . , xn . . . } −→ {ϕ0, ϕ1, . . . , ϕn . . . } ON polynomials.
E.g. Legendre-, Chebishev-, Hermite-polynomials. Do you know?
Questions.
I Why are these systems of orthogonal polynomials important?
I What can we use the ON polynomials for?
Functional analysis Distant Learning. Week 3.
A detour
Theorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions.
??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) ,
with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx ,
bk =1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary.
The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system
is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
A detourTheorem. (Classical Fourier theorem.)
Assume f : [−π, π]→ IR satisfies the Dirichlet conditions. ??
Then ∀x ∈ [−π, π]:
f (x) =a0
2+∞∑
k=1
(ak cos(kx) + bk sin(kx)) , with
ak =1π
∫ π
−πf (x) cos(kx) dx , bk =
1π
∫ π
−πf (x) sin(kx) dx .
Corollary. The trig. system is complete in L2[−π, π].
−→ Moreover, the coefficients are known.
Functional analysis Distant Learning. Week 3.
General Fourier series
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉.
I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H:
∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
In a Hilbert space.
(H, 〈·, ·〉) is a Hilbert space. (Can you recall the definition?)
Let (ϕk , ) ⊂ H be an ON system.
Theorem. Assume, that for some f ∈ H we have
f =∞∑
k=1
ckϕk .
Then ck = 〈f , ϕk 〉. I.e. the coefficients can be recovered from f .
Remark. If (ϕn) ⊂ H is complete, then every f ∈ H: ∃(cn)
f =∞∑
n=1
cnϕn.
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0.
(Why?) =⇒ 〈f , ϕj〉 = limn→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?)
=⇒ 〈f , ϕj〉 = limn→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩=
??? =n∑
k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ???
=n∑
k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉
= cj .
Functional analysis Distant Learning. Week 3.
Proof.
Let us define sn :=n∑
k=1
ckϕk .
Then by the Thm.’s assumption
limn→∞
‖f − sn‖ = 0.
It follows, that for all ϕj , j ≤ n
limn→∞〈f − sn, ϕj〉 = 0. (Why?) =⇒ 〈f , ϕj〉 = lim
n→∞〈sn, ϕj〉
If n ≥ j , then
〈sn, ϕj〉 =
⟨n∑
k=1
ckϕk , ϕj
⟩= ??? =
n∑k=1
ck 〈ϕk , ϕj〉 = cj .
Functional analysis Distant Learning. Week 3.
Fourier series expansion
Let (ϕn) ⊂ H be a complete ON system. For any f ∈ H we define
I FOURIER COEFFICIENTS of f with respect to (ϕn) as
〈f , ϕn〉 , n = 1,2, . . .
I FOURIER SERIES EXPANSION of f with respect to (ϕn) as
∞∑n=1
〈f , ϕn〉 ϕn.
Notation. f ∼∞∑
n=1
cn ϕn, with cn = 〈f , ϕn〉.
It is a formal definition yet. Why?
Functional analysis Distant Learning. Week 3.
Fourier series expansion
Let (ϕn) ⊂ H be a complete ON system. For any f ∈ H we define
I FOURIER COEFFICIENTS of f with respect to (ϕn) as
〈f , ϕn〉 , n = 1,2, . . .
I FOURIER SERIES EXPANSION of f with respect to (ϕn) as
∞∑n=1
〈f , ϕn〉 ϕn.
Notation. f ∼∞∑
n=1
cn ϕn, with cn = 〈f , ϕn〉.
It is a formal definition yet. Why?
Functional analysis Distant Learning. Week 3.
Fourier series expansion
Let (ϕn) ⊂ H be a complete ON system. For any f ∈ H we define
I FOURIER COEFFICIENTS of f with respect to (ϕn) as
〈f , ϕn〉 , n = 1,2, . . .
I FOURIER SERIES EXPANSION of f with respect to (ϕn) as
∞∑n=1
〈f , ϕn〉 ϕn.
Notation. f ∼∞∑
n=1
cn ϕn, with cn = 〈f , ϕn〉.
It is a formal definition yet. Why?
Functional analysis Distant Learning. Week 3.
Fourier series expansion
Let (ϕn) ⊂ H be a complete ON system. For any f ∈ H we define
I FOURIER COEFFICIENTS of f with respect to (ϕn) as
〈f , ϕn〉 , n = 1,2, . . .
I FOURIER SERIES EXPANSION of f with respect to (ϕn) as
∞∑n=1
〈f , ϕn〉 ϕn.
Notation. f ∼∞∑
n=1
cn ϕn, with cn = 〈f , ϕn〉.
It is a formal definition yet. Why?
Functional analysis Distant Learning. Week 3.
Fourier series expansion
Let (ϕn) ⊂ H be a complete ON system. For any f ∈ H we define
I FOURIER COEFFICIENTS of f with respect to (ϕn) as
〈f , ϕn〉 , n = 1,2, . . .
I FOURIER SERIES EXPANSION of f with respect to (ϕn) as
∞∑n=1
〈f , ϕn〉 ϕn.
Notation. f ∼∞∑
n=1
cn ϕn, with cn = 〈f , ϕn〉.
It is a formal definition yet. Why?
Functional analysis Distant Learning. Week 3.
Fourier series expansion
Let (ϕn) ⊂ H be a complete ON system. For any f ∈ H we define
I FOURIER COEFFICIENTS of f with respect to (ϕn) as
〈f , ϕn〉 , n = 1,2, . . .
I FOURIER SERIES EXPANSION of f with respect to (ϕn) as
∞∑n=1
〈f , ϕn〉 ϕn.
Notation. f ∼∞∑
n=1
cn ϕn, with cn = 〈f , ϕn〉.
It is a formal definition yet.
Why?
Functional analysis Distant Learning. Week 3.
Fourier series expansion
Let (ϕn) ⊂ H be a complete ON system. For any f ∈ H we define
I FOURIER COEFFICIENTS of f with respect to (ϕn) as
〈f , ϕn〉 , n = 1,2, . . .
I FOURIER SERIES EXPANSION of f with respect to (ϕn) as
∞∑n=1
〈f , ϕn〉 ϕn.
Notation. f ∼∞∑
n=1
cn ϕn, with cn = 〈f , ϕn〉.
It is a formal definition yet. Why?
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy.
V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space.
v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space
basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Sum of the Fourier series
Theorem. If (ϕn) is a complete ON system, then
f =∞∑
n=1
〈f , ϕn〉 ϕn.
I.e. the sum of the Fourer series gives back the original function.
Analogy. V is a finite dim. vector space. v1, . . . , vn ∈ V is a basis, if
I these vectors are linearly independent,
I ∀v ∈ V can be written as v =n∑
k=1
ck vk (i.e. a generator system).
In infinite dimensional Hilbert space basis ≡ complete ON system
Functional analysis Distant Learning. Week 3.
Parseval equality
Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem.
Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system.
Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2,
cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete
⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒
∞∑n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
Parseval equality Try to recall ”the original” one
Theorem. Let f ∈ H.
1. (ϕn) ⊂ H is an ON system. Then
∞∑n=1
c2n ≤ ‖f‖2, cn = 〈f , ϕn〉 ϕn.
2. (ϕn) is ON and complete ⇐⇒∞∑
n=1
c2n = ‖f‖2.
The latter identity is called PARSEVAL EQUALITY.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2
cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof.
1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk .
Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is
try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...
the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}.
Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2
=⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =
n∑k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k .
Finally, with n→∞√
.
Functional analysis Distant Learning. Week 3.
∞∑n=1
c2n ≤ ‖f‖2 cn = 〈f , ϕn〉 ϕn.,
Proof. 1. Let us define sn :=n∑
k=1
ckϕk . Geometrically it is try to
finish...the projection of f onto span{ϕ1, ..., ϕn}. Thus (f − sn)⊥sn.
Then we can use the Pythagorean theorem:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2 =⇒ ‖sn‖2 ≤ ‖f‖2 ∀n.
By orthogonality ‖sn‖2 =n∑
k=1
c2k . Finally, with n→∞
√.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2
⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON.
From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0.
From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =
∞∑k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B.
Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =
∞∑k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f ,
prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE.
Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.∞∑
n=1
c2n = ‖f‖2 ⇐⇒ (ϕn) is complete,
2. To verify a proposition with ⇐⇒ inside has to parts.
Part A. Assume (ϕn) is ON. From the previous slide:
‖f‖2 = ‖f − sn‖2 + ‖sn‖2. (1)
From the completeness of (ϕn) follows, that f =∞∑
n=1
cnϕn, thus
limn→∞
‖f − sn‖2 = 0. From (1) we get ‖f‖2 = limn→∞
‖sn‖2 =∞∑
k=1
c2k .
Part B. Assuming ‖f‖2 =∞∑
k=1
c2k ∀f , prove (ϕn) is COMPLETE. Do it
Yourself. HW.
Functional analysis Distant Learning. Week 3.
Generalized Parseval equality.
Theorem. Let (ϕn) be a complete ON system in L2(R)-ben.
f ,g ∈ L2(R) are arbitrary functions.Then
〈f ,g〉 =∞∑
k=1
ck dk ,
where c = (ck ) and d = (dk ) are the Fourier coefficients of f and g.
This relation can be also written as:
〈f ,g〉L2 = 〈c,d〉`2 .
Functional analysis Distant Learning. Week 3.
Generalized Parseval equality.
Theorem. Let (ϕn) be a complete ON system in L2(R)-ben.
f ,g ∈ L2(R) are arbitrary functions.Then
〈f ,g〉 =∞∑
k=1
ck dk ,
where c = (ck ) and d = (dk ) are the Fourier coefficients of f and g.
This relation can be also written as:
〈f ,g〉L2 = 〈c,d〉`2 .
Functional analysis Distant Learning. Week 3.
Generalized Parseval equality.
Theorem. Let (ϕn) be a complete ON system in L2(R)-ben.
f ,g ∈ L2(R) are arbitrary functions.
Then
〈f ,g〉 =∞∑
k=1
ck dk ,
where c = (ck ) and d = (dk ) are the Fourier coefficients of f and g.
This relation can be also written as:
〈f ,g〉L2 = 〈c,d〉`2 .
Functional analysis Distant Learning. Week 3.
Generalized Parseval equality.
Theorem. Let (ϕn) be a complete ON system in L2(R)-ben.
f ,g ∈ L2(R) are arbitrary functions.Then
〈f ,g〉 =∞∑
k=1
ck dk ,
where c = (ck ) and d = (dk ) are the Fourier coefficients of f and g.
This relation can be also written as:
〈f ,g〉L2 = 〈c,d〉`2 .
Functional analysis Distant Learning. Week 3.
Generalized Parseval equality.
Theorem. Let (ϕn) be a complete ON system in L2(R)-ben.
f ,g ∈ L2(R) are arbitrary functions.Then
〈f ,g〉 =∞∑
k=1
ck dk ,
where c = (ck ) and d = (dk ) are the Fourier coefficients of f and g.
This relation can be also written as:
〈f ,g〉L2 = 〈c,d〉`2 .
Functional analysis Distant Learning. Week 3.
Generalized Parseval equality.
Theorem. Let (ϕn) be a complete ON system in L2(R)-ben.
f ,g ∈ L2(R) are arbitrary functions.Then
〈f ,g〉 =∞∑
k=1
ck dk ,
where c = (ck ) and d = (dk ) are the Fourier coefficients of f and g.
This relation can be also written as:
〈f ,g〉L2 =
〈c,d〉`2 .
Functional analysis Distant Learning. Week 3.
Generalized Parseval equality.
Theorem. Let (ϕn) be a complete ON system in L2(R)-ben.
f ,g ∈ L2(R) are arbitrary functions.Then
〈f ,g〉 =∞∑
k=1
ck dk ,
where c = (ck ) and d = (dk ) are the Fourier coefficients of f and g.
This relation can be also written as:
〈f ,g〉L2 = 〈c,d〉`2 .
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint) A ”candidate” is f :=∞∑
k=1
dkϕk . It is OK. Finish the proof.
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint) A ”candidate” is f :=∞∑
k=1
dkϕk . It is OK. Finish the proof.
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint) A ”candidate” is f :=∞∑
k=1
dkϕk . It is OK. Finish the proof.
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint) A ”candidate” is f :=∞∑
k=1
dkϕk . It is OK. Finish the proof.
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint)
A ”candidate” is f :=∞∑
k=1
dkϕk . It is OK. Finish the proof.
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint) A ”candidate” is f :=∞∑
k=1
dkϕk .
It is OK. Finish the proof.
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint) A ”candidate” is f :=∞∑
k=1
dkϕk . It is OK.
Finish the proof.
Functional analysis Distant Learning. Week 3.
Special case: H = L2(R)
Corollary. For any f ∈ L2(R) it is possible to assign (cn) ∈ `2, usingany (ϕn) complete ON system.
The other direction is the following important Thm.
Theorem. (Riesz-Fisher thm.) Let (dk ) ∈ `2, i.e.∞∑
k=1
d2k <∞.
Then ∃f ∈ L2(R), such that ‖f‖2 =∞∑
k=1
d2k , and it’s Fourier coefficients
are dk .
Proof. (Hint) A ”candidate” is f :=∞∑
k=1
dkϕk . It is OK. Finish the proof.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients: f ←→ (cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients: f ←→ (cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients:
f ←→ (cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients:
f ←→ (cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients: f ←→
(cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients: f ←→ (cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients: f ←→ (cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
L2 and `2
Corollary. L2(R) es `2 are isometrically isomorphic.
The linear isometry is based an any (ϕn) complete ON system,
using the Fourier coefficients: f ←→ (cn).
PLEASE STOP FOR A WHILE, AND UNDERSTAND THIS POINT.
L2(R) and `2 are the ”same”.
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2,
P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) =
it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients.
Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
Example. H = L2[−1,1]
In L2[−1,1] a complete ON system are the Legendre polynomials.
We have seen some elements of (Pn(x)):
P0(x) =1√2, P1(x) =
√32
x , P2(x) = it was a HW . . . ..
Then every f ∈ L2[−1,1] can be written as
f (x) =∞∑
n=0
cnPn(x), with cn =
∫ 1
−1f (x)Pn(x)dx .
Thus every f ∈ L2[−1,1] can be approximated by a polynomial of
degree n with KNOWN coefficients. Can you recall sg. similar?
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
( More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
( More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
(
More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
( More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
( More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
( More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k
with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
( More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
An example in H = L2[0,1]
This example gives an ON system in L2[0,1]-ben.
They are called Haar-functions.
They are not polynomials, but this is the simplest wavelet family .
( More details on that can be found in the in the book.)
They are defined in blocks.
Hn,k with n = 0,1,2, . . . k = 1, ...,2n.
For all indices Hn,k : [0,1]→ IR.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and H0,0⊥H0,1. DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and H0,0⊥H0,1. DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and H0,0⊥H0,1. DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]
This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and H0,0⊥H0,1. DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and H0,0⊥H0,1. DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and
H0,0⊥H0,1. DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and H0,0⊥H0,1.
DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions
For n = 0 there are two functions: H0,0 and H0,1.
H0,0(x) = 1.
H0,1(x) =
1 if 0 ≤ x < 1/2
−1 if 1/2 ≤ x ≤ 1
x ∈ [0,1]This is the so called mother wavelet
Easy to check, that ‖H0,0‖ = ‖H0,1‖ = 1 and H0,0⊥H0,1. DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n .
Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and
Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k .
DO IT.
Functional analysis Distant Learning. Week 3.
Haar functions, nth block.
For n ≥ 1 divide [0,1] into 2n equal parts with pointsk2n . Let’s define:
Hn,k (x) =
√2n if
k − 12n ≤ x <
k − 1/22n
−√
2n ifk − 1/2
2n ≤ x <k2n
0 otherwise
, n ≥ 1, 1 ≤ k ≤ 2n.
The nonzero part is the ”mother wavelet”, squished and stretched.
Easy to check, that ‖Hn,k‖ = 1 and Hn,k⊥Hn,j for j 6= k . DO IT.
Functional analysis Distant Learning. Week 3.
E.g. Haar functions H2,k
As an example, hereare the graphs of the
H2,k
Haar functions fork = 1,2,3,4.
Remark. This ON system is complete. (Not trivial to prove. )
Functional analysis Distant Learning. Week 3.
E.g. Haar functions H2,k
As an example, hereare the graphs of the
H2,k
Haar functions fork = 1,2,3,4.
Remark. This ON system is complete. (Not trivial to prove. )
Functional analysis Distant Learning. Week 3.
E.g. Haar functions H2,k
As an example, hereare the graphs of the
H2,k
Haar functions fork = 1,2,3,4.
Remark. This ON system is complete. (Not trivial to prove. )
Functional analysis Distant Learning. Week 3.
E.g. Haar functions H2,k
As an example, hereare the graphs of the
H2,k
Haar functions fork = 1,2,3,4.
Remark. This ON system is complete. (Not trivial to prove. )