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Outlines
Life of π: Continued Fractions and Infinite Series
Daniel J. Hermes
February 29, 2012
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Outlines
π = 3 +1
6 +9
6 +25
6 +49
6 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
OutlinesPart I: Introductory FactsPart II: What We Came Here For: π
Outline of Part I
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
OutlinesPart I: Introductory FactsPart II: What We Came Here For: π
Outline of Part I
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
OutlinesPart I: Introductory FactsPart II: What We Came Here For: π
Outline of Part I
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
OutlinesPart I: Introductory FactsPart II: What We Came Here For: π
Outline of Part I
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
OutlinesPart I: Introductory FactsPart II: What We Came Here For: π
Outline of Part II
Analysis and ArithmeticA Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Parting Words4π and 12
π2
Thank You
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
OutlinesPart I: Introductory FactsPart II: What We Came Here For: π
Outline of Part II
Analysis and ArithmeticA Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Parting Words4π and 12
π2
Thank You
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Part I
Introductory Facts
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
The Wrong Way
In bill #246 of the 1897 sitting of the Indiana General Assembly, πwas rational:
...Furthermore, it has revealed the ratio of the chord andarc of ninety degrees, which is as seven to eight, and alsothe ratio of the diagonal and one side of a square whichis as ten to seven, disclosing the fourth important fact,that the ratio of the diameter and circumference is asfive-fourths to four...
π =4
5/4= 3.2
http://en.wikipedia.org/wiki/Indiana Pi Bill
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
The Wrong Way
In bill #246 of the 1897 sitting of the Indiana General Assembly, πwas rational:
...Furthermore, it has revealed the ratio of the chord andarc of ninety degrees, which is as seven to eight, and alsothe ratio of the diagonal and one side of a square whichis as ten to seven, disclosing the fourth important fact,that the ratio of the diameter and circumference is asfive-fourths to four...
π =4
5/4= 3.2
http://en.wikipedia.org/wiki/Indiana Pi Bill
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
The Wrong Way
In bill #246 of the 1897 sitting of the Indiana General Assembly, πwas rational:
...Furthermore, it has revealed the ratio of the chord andarc of ninety degrees, which is as seven to eight, and alsothe ratio of the diagonal and one side of a square whichis as ten to seven, disclosing the fourth important fact,that the ratio of the diameter and circumference is asfive-fourths to four...
π =4
5/4= 3.2
http://en.wikipedia.org/wiki/Indiana Pi Bill
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: Archimedes
In the third century BCE, Archimedes proved the sharpinequalities
223/71 < π < 22/7
by means of regular 96-gons
22
7= 3 +
1
7
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: Archimedes
In the third century BCE, Archimedes proved the sharpinequalities
223/71 < π < 22/7
by means of regular 96-gons
22
7= 3 +
1
7
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: The Bible
In 1 Kings 7:23 the word translated ’measuring line’appears in the Hebrew text ... The ratio of the numericalvalues of these Hebrew spellings is 111/106. If the putativevalue of 3 is multiplied by this ratio, one obtains 333/106
333
106= 3 +
15
106
= 3 +1
106/15
= 3 +1
7 +1
15
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: The Bible
In 1 Kings 7:23 the word translated ’measuring line’appears in the Hebrew text ... The ratio of the numericalvalues of these Hebrew spellings is 111/106. If the putativevalue of 3 is multiplied by this ratio, one obtains 333/106
333
106= 3 +
15
106
= 3 +1
106/15
= 3 +1
7 +1
15
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: The Bible
In 1 Kings 7:23 the word translated ’measuring line’appears in the Hebrew text ... The ratio of the numericalvalues of these Hebrew spellings is 111/106. If the putativevalue of 3 is multiplied by this ratio, one obtains 333/106
333
106= 3 +
15
106
= 3 +1
106/15
= 3 +1
7 +1
15
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: The Bible
In 1 Kings 7:23 the word translated ’measuring line’appears in the Hebrew text ... The ratio of the numericalvalues of these Hebrew spellings is 111/106. If the putativevalue of 3 is multiplied by this ratio, one obtains 333/106
333
106= 3 +
15
106
= 3 +1
106/15
= 3 +1
7 +1
15
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: Zu Chongzhi
The 5th century Chinese mathematician and astronomerZu Chongzhi ... gave two other approximations ... 22/7and 355/113
355
113= 3 +
16
113
= 3 +1
113/16= 3 +
1
7 + 1/16
= 3 +1
7 +1
15 + 1/1
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: Zu Chongzhi
The 5th century Chinese mathematician and astronomerZu Chongzhi ... gave two other approximations ... 22/7and 355/113
355
113= 3 +
16
113
= 3 +1
113/16= 3 +
1
7 + 1/16
= 3 +1
7 +1
15 + 1/1
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: Zu Chongzhi
The 5th century Chinese mathematician and astronomerZu Chongzhi ... gave two other approximations ... 22/7and 355/113
355
113= 3 +
16
113
= 3 +1
113/16= 3 +
1
7 + 1/16
= 3 +1
7 +1
15 + 1/1
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: Zu Chongzhi
The 5th century Chinese mathematician and astronomerZu Chongzhi ... gave two other approximations ... 22/7and 355/113
355
113= 3 +
16
113
= 3 +1
113/16= 3 +
1
7 + 1/16
= 3 +1
7 +1
15 + 1/1
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: James Gregory
In 1672, James Gregory wrote about a formula for calculating theangle given the tangent x :
arctan x = x − x3
3+
x5
5− x7
7+ · · ·
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: James Gregory
In 1672, James Gregory wrote about a formula for calculating theangle given the tangent x :
arctan x = x − x3
3+
x5
5− x7
7+ · · ·
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: James Gregory
Why is this relevant? We can use it to approximate π!
π
4= 1− 1
3+
1
5− 1
7+ · · · =
∞∑n=0
(−1)n
2n + 1
Proof: Since ddx arctan x =
1
1 + x2= 1− x2 + x4 − x6 + · · · and
arctan 0 = 0, integrating, we find the Taylor series expansion forarctan is
arctan x = x − x3
3+
x5
5− x7
7+ · · ·
Henceπ
4= arctan(1) =
∞∑n=0
(−1)n
2n + 1. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: James Gregory
Why is this relevant? We can use it to approximate π!
π
4= 1− 1
3+
1
5− 1
7+ · · · =
∞∑n=0
(−1)n
2n + 1
Proof: Since ddx arctan x =
1
1 + x2= 1− x2 + x4 − x6 + · · · and
arctan 0 = 0, integrating, we find the Taylor series expansion forarctan is
arctan x = x − x3
3+
x5
5− x7
7+ · · ·
Henceπ
4= arctan(1) =
∞∑n=0
(−1)n
2n + 1. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: James Gregory
Why is this relevant? We can use it to approximate π!
π
4= 1− 1
3+
1
5− 1
7+ · · · =
∞∑n=0
(−1)n
2n + 1
Proof: Since ddx arctan x =
1
1 + x2= 1− x2 + x4 − x6 + · · · and
arctan 0 = 0, integrating, we find the Taylor series expansion forarctan is
arctan x = x − x3
3+
x5
5− x7
7+ · · ·
Henceπ
4= arctan(1) =
∞∑n=0
(−1)n
2n + 1. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: James Gregory
Why is this relevant? We can use it to approximate π!
π
4= 1− 1
3+
1
5− 1
7+ · · · =
∞∑n=0
(−1)n
2n + 1
Proof: Since ddx arctan x =
1
1 + x2= 1− x2 + x4 − x6 + · · · and
arctan 0 = 0, integrating, we find the Taylor series expansion forarctan is
arctan x = x − x3
3+
x5
5− x7
7+ · · ·
Henceπ
4= arctan(1) =
∞∑n=0
(−1)n
2n + 1. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Smart Men: James Gregory
Why is this relevant? We can use it to approximate π!
π
4= 1− 1
3+
1
5− 1
7+ · · · =
∞∑n=0
(−1)n
2n + 1
Proof: Since ddx arctan x =
1
1 + x2= 1− x2 + x4 − x6 + · · · and
arctan 0 = 0, integrating, we find the Taylor series expansion forarctan is
arctan x = x − x3
3+
x5
5− x7
7+ · · ·
Henceπ
4= arctan(1) =
∞∑n=0
(−1)n
2n + 1. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
The Wrong WaySmart MenConway and Pell
Conway and Pell
In the actual talk, I displayed “Hey Dan, go to the chalkboardplease.” and just talked to them. For some reference see:
I blog.bossylobster.com/2011/07/continued-fractions-for-greater-good.html
I blog.bossylobster.com/2011/07/continued-fraction-expansions-of.html
I blog.bossylobster.com/2011/08/conways-topograph-part-1.html
I blog.bossylobster.com/2011/08/conways-topograph-part-2.html
I blog.bossylobster.com/2011/08/conways-topograph-part-3.html
I blog.bossylobster.com/2011/08/finding-fibonacci-golden-nuggets.html
I blog.bossylobster.com/2011/08/finding-fibonacci-golden-nuggets-part-2.html
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
Define a standard continued fraction with a series of nonnegativeintegers {an}∞n=0:
a0 +1
a1 +1
a2 +1
a3 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
Define a standard continued fraction with a series of nonnegativeintegers {an}∞n=0:
a0 +1
a1 +1
a2 +1
a3 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
How will we know if our fractions converge if we don’t have aconcept of a “convergent sum”?
For a standard continued fractioncorresponding to {an}, let the partial convergent “stopping” at anbe the fraction hn/kn. As we’ll show, the series {hn} and {kn} arevery related and can be used to determine convergence.
I Notice the zeroth partial is a0 so h0 = a0, k0 = 1.
I The first is a0 + 1a1
= a1(a0)+1a1(1)+0 , h1 = a1a0 + 1, k1 = a1.
I The second is a0 +1
a1 +1
a2
= a0 + a2a1a2+1 = a2(a0a1+1)+a0
a2(a1)+1 .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
How will we know if our fractions converge if we don’t have aconcept of a “convergent sum”? For a standard continued fractioncorresponding to {an}, let the partial convergent “stopping” at anbe the fraction hn/kn.
As we’ll show, the series {hn} and {kn} arevery related and can be used to determine convergence.
I Notice the zeroth partial is a0 so h0 = a0, k0 = 1.
I The first is a0 + 1a1
= a1(a0)+1a1(1)+0 , h1 = a1a0 + 1, k1 = a1.
I The second is a0 +1
a1 +1
a2
= a0 + a2a1a2+1 = a2(a0a1+1)+a0
a2(a1)+1 .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
How will we know if our fractions converge if we don’t have aconcept of a “convergent sum”? For a standard continued fractioncorresponding to {an}, let the partial convergent “stopping” at anbe the fraction hn/kn. As we’ll show, the series {hn} and {kn} arevery related and can be used to determine convergence.
I Notice the zeroth partial is a0 so h0 = a0, k0 = 1.
I The first is a0 + 1a1
= a1(a0)+1a1(1)+0 , h1 = a1a0 + 1, k1 = a1.
I The second is a0 +1
a1 +1
a2
= a0 + a2a1a2+1 = a2(a0a1+1)+a0
a2(a1)+1 .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
How will we know if our fractions converge if we don’t have aconcept of a “convergent sum”? For a standard continued fractioncorresponding to {an}, let the partial convergent “stopping” at anbe the fraction hn/kn. As we’ll show, the series {hn} and {kn} arevery related and can be used to determine convergence.
I Notice the zeroth partial is a0 so h0 = a0, k0 = 1.
I The first is a0 + 1a1
= a1(a0)+1a1(1)+0 , h1 = a1a0 + 1, k1 = a1.
I The second is a0 +1
a1 +1
a2
= a0 + a2a1a2+1 = a2(a0a1+1)+a0
a2(a1)+1 .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
How will we know if our fractions converge if we don’t have aconcept of a “convergent sum”? For a standard continued fractioncorresponding to {an}, let the partial convergent “stopping” at anbe the fraction hn/kn. As we’ll show, the series {hn} and {kn} arevery related and can be used to determine convergence.
I Notice the zeroth partial is a0 so h0 = a0, k0 = 1.
I The first is a0 + 1a1
= a1(a0)+1a1(1)+0 , h1 = a1a0 + 1, k1 = a1.
I The second is a0 +1
a1 +1
a2
= a0 + a2a1a2+1 = a2(a0a1+1)+a0
a2(a1)+1 .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Define It
How will we know if our fractions converge if we don’t have aconcept of a “convergent sum”? For a standard continued fractioncorresponding to {an}, let the partial convergent “stopping” at anbe the fraction hn/kn. As we’ll show, the series {hn} and {kn} arevery related and can be used to determine convergence.
I Notice the zeroth partial is a0 so h0 = a0, k0 = 1.
I The first is a0 + 1a1
= a1(a0)+1a1(1)+0 , h1 = a1a0 + 1, k1 = a1.
I The second is a0 +1
a1 +1
a2
= a0 + a2a1a2+1 = a2(a0a1+1)+a0
a2(a1)+1 .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
What Does it Mean to Converge?
Claim: hn and kn satisfy the same recurrence relation
hn = anhn−1 + hn−2
kn = ankn−1 + kn−2
along with initial conditions h−1 = 1, h−2 = 0 andk−1 = 0, k−2 = 1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
What Does it Mean to Converge?
Proof: There is one key insight: when go from one partial to the
next by turning the final 1an
into1
an + 1an+1
. Using the recurrence
and the inductive assumption:
hn
kn=
anhn−1 + hn−2
ankn−1 + kn−2
becomeshn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
Why can we replace an straight-up? All the other terms dependsolely on a1, . . . , an−1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
What Does it Mean to Converge?
Proof: There is one key insight: when go from one partial to the
next by turning the final 1an
into1
an + 1an+1
. Using the recurrence
and the inductive assumption:
hn
kn=
anhn−1 + hn−2
ankn−1 + kn−2
becomeshn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
Why can we replace an straight-up? All the other terms dependsolely on a1, . . . , an−1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
What Does it Mean to Converge?
Proof: There is one key insight: when go from one partial to the
next by turning the final 1an
into1
an + 1an+1
. Using the recurrence
and the inductive assumption:
hn
kn=
anhn−1 + hn−2
ankn−1 + kn−2
becomeshn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
Why can we replace an straight-up? All the other terms dependsolely on a1, . . . , an−1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
What Does it Mean to Converge?
Proof: There is one key insight: when go from one partial to the
next by turning the final 1an
into1
an + 1an+1
. Using the recurrence
and the inductive assumption:
hn
kn=
anhn−1 + hn−2
ankn−1 + kn−2
becomeshn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
Why can we replace an straight-up?
All the other terms dependsolely on a1, . . . , an−1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
What Does it Mean to Converge?
Proof: There is one key insight: when go from one partial to the
next by turning the final 1an
into1
an + 1an+1
. Using the recurrence
and the inductive assumption:
hn
kn=
anhn−1 + hn−2
ankn−1 + kn−2
becomeshn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
Why can we replace an straight-up? All the other terms dependsolely on a1, . . . , an−1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Standard
andhn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
=an+1(anhn−1 + hn−2) + hn−1
an+1(ankn−1 + kn−2) + kn−1
=an+1hn + hn−1
an+1kn + kn−1by the inductive assumption.
Clearly, the recurrence is determined by just two terms, so with asimple check, h−1 = 1, h−2 = 0 and k−1 = 0, k−1 = 1⇒ h0 = a0, h1 = a0a1 + 1 and k0 = 1, k1 = a1 as we’d wish. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Standard
andhn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
=an+1(anhn−1 + hn−2) + hn−1
an+1(ankn−1 + kn−2) + kn−1
=an+1hn + hn−1
an+1kn + kn−1by the inductive assumption.
Clearly, the recurrence is determined by just two terms, so with asimple check, h−1 = 1, h−2 = 0 and k−1 = 0, k−1 = 1⇒ h0 = a0, h1 = a0a1 + 1 and k0 = 1, k1 = a1 as we’d wish. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Standard
andhn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
=an+1(anhn−1 + hn−2) + hn−1
an+1(ankn−1 + kn−2) + kn−1
=an+1hn + hn−1
an+1kn + kn−1by the inductive assumption.
Clearly, the recurrence is determined by just two terms, so with asimple check, h−1 = 1, h−2 = 0 and k−1 = 0, k−1 = 1⇒ h0 = a0, h1 = a0a1 + 1 and k0 = 1, k1 = a1 as we’d wish. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
Standard
andhn+1
kn+1=
(an + 1
an+1
)hn−1 + hn−2(
an + 1an+1
)kn−1 + kn−2
=an+1(anhn−1 + hn−2) + hn−1
an+1(ankn−1 + kn−2) + kn−1
=an+1hn + hn−1
an+1kn + kn−1by the inductive assumption.
Clearly, the recurrence is determined by just two terms, so with asimple check, h−1 = 1, h−2 = 0 and k−1 = 0, k−1 = 1⇒ h0 = a0, h1 = a0a1 + 1 and k0 = 1, k1 = a1 as we’d wish. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
General
If we look to our favorite Sloane number sequence (A001203), wefind that standard continued fractions don’t do anything helpfulwith π:
π = 3 +1
7 +1
15 +1
1 +1
292 +. . .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
General
If we look to our favorite Sloane number sequence (A001203), wefind that standard continued fractions don’t do anything helpfulwith π:
π = 3 +1
7 +1
15 +1
1 +1
292 +. . .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
General
But...
π = 3 +1
6 +9
6 +25
6 +49
6 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
General
But...
π = 3 +1
6 +9
6 +25
6 +49
6 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
General
Let’s generalize our definition!
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
GeneralToo General?
Define a generalized continued fraction with two series ofnonnegative integers {an}∞n=0 and {bn}∞n=0:
a0 +b1
a1 +b2
a2 +b3
a3 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
GeneralToo General?
Define a generalized continued fraction with two series ofnonnegative integers {an}∞n=0 and {bn}∞n=0:
a0 +b1
a1 +b2
a2 +b3
a3 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
GeneralToo General?
Again, let the partial convergent “stopping” at an be the fractionhn/kn.
Claim: hn and kn satisfy the same recurrence relation
hn = anhn−1 + bnhn−2
kn = ankn−1 + bnkn−2
along with initial conditions h−1 = 1, h−2 = 0 andk−1 = 0, k−2 = 1.
We won’t prove it, but the key insight is (you guessed it) turningan into an + bn+1
an+1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
GeneralToo General?
Again, let the partial convergent “stopping” at an be the fractionhn/kn.
Claim: hn and kn satisfy the same recurrence relation
hn = anhn−1 + bnhn−2
kn = ankn−1 + bnkn−2
along with initial conditions h−1 = 1, h−2 = 0 andk−1 = 0, k−2 = 1.
We won’t prove it, but the key insight is (you guessed it) turningan into an + bn+1
an+1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
Define ItWhat Does it Mean to Converge?General
GeneralToo General?
Again, let the partial convergent “stopping” at an be the fractionhn/kn.
Claim: hn and kn satisfy the same recurrence relation
hn = anhn−1 + bnhn−2
kn = ankn−1 + bnkn−2
along with initial conditions h−1 = 1, h−2 = 0 andk−1 = 0, k−2 = 1.
We won’t prove it, but the key insight is (you guessed it) turningan into an + bn+1
an+1.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
In the actual talk, I displayed “Hey Dan, go back to the chalkboardplease.” and just talked to them. I have included it here since
there is no chalkboard.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
), with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
)
, with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
), with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
), with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
), with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
), with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
), with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Difference: Dn
We wish to find the value of the continued fraction: limn→∞
hn
kn.
Noticeh1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
), with this it becomes clear it
is important to define the difference: Dn+1 = kn+1hn − knhn+1.
We have D1 = k1h0 − k0h1 = a1a0 − 1 · (a1a0 + b1) = −b1 and
Dn+1 = (an+1kn + bn+1kn−1)hn − kn(an+1hn + bn+1hn−1)
= bn+1(kn−1hn − knhn−1)
= −bn+1Dn
so any easy induction gives us Dn = (−1)n∏n
r=1 br .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Summary: Dn
After I returned from the chalkboard, I used this slide.
To summarize what we just said:
Dn+1 = kn+1hn − knhn+1
Dn = (−1)nn∏
r=1
br
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Summary: Dn
After I returned from the chalkboard, I used this slide.To summarize what we just said:
Dn+1 = kn+1hn − knhn+1
Dn = (−1)nn∏
r=1
br
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Telescoping Partials
Again, in the actual talk, I displayed “Hey Dan, go back to thechalkboard please.” and just talked to them. I have included it
here since there is no chalkboard.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Telescoping Partials
For the partials:hn
kn= a0 −
n∑r=1
Dr
krkr−1
h1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
)
=n−1∑r=1
Dr+1
krkr+1but D1 = −b1 = a1a0 − h1
⇒ hn
kn=
h1
k1−
n−1∑r=1
Dr+1
krkr+1= a0 −
n∑r=1
Dr
krkr−1
since k0 = 1, k1 = a1 ⇒ h1k1
= a1a0−D1k1
= a1a0a1− D1
k1k0.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Telescoping Partials
For the partials:hn
kn= a0 −
n∑r=1
Dr
krkr−1
h1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
)
=n−1∑r=1
Dr+1
krkr+1but D1 = −b1 = a1a0 − h1
⇒ hn
kn=
h1
k1−
n−1∑r=1
Dr+1
krkr+1= a0 −
n∑r=1
Dr
krkr−1
since k0 = 1, k1 = a1 ⇒ h1k1
= a1a0−D1k1
= a1a0a1− D1
k1k0.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Telescoping Partials
For the partials:hn
kn= a0 −
n∑r=1
Dr
krkr−1
h1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
)
=n−1∑r=1
Dr+1
krkr+1but D1 = −b1 = a1a0 − h1
⇒ hn
kn=
h1
k1−
n−1∑r=1
Dr+1
krkr+1= a0 −
n∑r=1
Dr
krkr−1
since k0 = 1, k1 = a1 ⇒ h1k1
= a1a0−D1k1
= a1a0a1− D1
k1k0.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Telescoping Partials
For the partials:hn
kn= a0 −
n∑r=1
Dr
krkr−1
h1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
)
=n−1∑r=1
Dr+1
krkr+1but D1 = −b1 = a1a0 − h1
⇒ hn
kn=
h1
k1−
n−1∑r=1
Dr+1
krkr+1= a0 −
n∑r=1
Dr
krkr−1
since k0 = 1, k1 = a1 ⇒ h1k1
= a1a0−D1k1
= a1a0a1− D1
k1k0.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Telescoping Partials
For the partials:hn
kn= a0 −
n∑r=1
Dr
krkr−1
h1
k1− hn
kn=
n−1∑r=1
(hr
kr− hr+1
kr+1
)
=n−1∑r=1
Dr+1
krkr+1but D1 = −b1 = a1a0 − h1
⇒ hn
kn=
h1
k1−
n−1∑r=1
Dr+1
krkr+1= a0 −
n∑r=1
Dr
krkr−1
since k0 = 1, k1 = a1 ⇒ h1k1
= a1a0−D1k1
= a1a0a1− D1
k1k0.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Summary: Partials
After I returned from the chalkboard, I used this slide.
hn
kn= a0 −
n∑r=1
Dr
krkr−1
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
DifferenceTelescoping Partials
Summary: Partials
After I returned from the chalkboard, I used this slide.
hn
kn= a0 −
n∑r=1
Dr
krkr−1
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
IntroductionThe Wrong WaySmart MenConway and Pell
Continued FractionsDefine ItWhat Does it Mean to Converge?General
Working with the ConvergentsDifferenceTelescoping Partials
Lemmata: Sums for πReviewSmith Sum
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
Review
Recall that we showed:
π
4= 1− 1
3+
1
5− 1
7+ · · · =
∞∑n=0
(−1)n
2n + 1
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
Smith Sum:This is equivalent to the previous sum
Claim:π − 3
4=∞∑n=1
(−1)n−1
(2n)(2n + 1)(2n + 2)
Proof: Here we just manipulate the summand using the partialfractal decomposition
1
2n(2n + 1)(2n + 2)=
1
4n− 1
2n + 1+
1
4n + 4.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
Smith Sum:This is equivalent to the previous sum
Claim:π − 3
4=∞∑n=1
(−1)n−1
(2n)(2n + 1)(2n + 2)
Proof: Here we just manipulate the summand using the partialfractal decomposition
1
2n(2n + 1)(2n + 2)=
1
4n− 1
2n + 1+
1
4n + 4.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
Smith Sum:
∞∑n=1
(−1)n−1
2n(2n + 1)(2n + 2)
=∞∑n=1
(−1)n−1
4n+∞∑n=1
(−1)n
2n + 1+∞∑n=1
(−1)n−1
4n + 4
=∞∑n=1
(−1)n−1
4n+∞∑n=0
(−1)n
2n + 1− 1 +
∞∑n=2
(−1)n
4n
=(−1)1+1
4 · 1+π
4− 1 =
π − 3
4�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
Smith Sum:
∞∑n=1
(−1)n−1
2n(2n + 1)(2n + 2)
=∞∑n=1
(−1)n−1
4n+∞∑n=1
(−1)n
2n + 1+∞∑n=1
(−1)n−1
4n + 4
=∞∑n=1
(−1)n−1
4n+∞∑n=0
(−1)n
2n + 1− 1 +
∞∑n=2
(−1)n
4n
=(−1)1+1
4 · 1+π
4− 1 =
π − 3
4�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
Smith Sum:
∞∑n=1
(−1)n−1
2n(2n + 1)(2n + 2)
=∞∑n=1
(−1)n−1
4n+∞∑n=1
(−1)n
2n + 1+∞∑n=1
(−1)n−1
4n + 4
=∞∑n=1
(−1)n−1
4n+∞∑n=0
(−1)n
2n + 1− 1 +
∞∑n=2
(−1)n
4n
=(−1)1+1
4 · 1+π
4− 1 =
π − 3
4�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
IntroCF
ConvergentsLemmata: Sums for π
ReviewSmith Sum
Smith Sum:
∞∑n=1
(−1)n−1
2n(2n + 1)(2n + 2)
=∞∑n=1
(−1)n−1
4n+∞∑n=1
(−1)n
2n + 1+∞∑n=1
(−1)n−1
4n + 4
=∞∑n=1
(−1)n−1
4n+∞∑n=0
(−1)n
2n + 1− 1 +
∞∑n=2
(−1)n
4n
=(−1)1+1
4 · 1+π
4− 1 =
π − 3
4�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
Part II
What We Came Here For: π
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Analysis and ArithmeticA Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Parting Words4π and 12
π2
Thank You
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
A Nifty Identity
If 1c1− 1
c2+ 1
c3− · · · =
∑∞n=1
(−1)n−1
cnconverges then so does the
fraction1
c1 +c2
1
c2 − c1 +c2
2
c3 − c2 +c2
3
c4 − c3 +.. .
and they are
equivalent, i.e. they are equal at every convergent.
Here a0 = 0, a1 = c1, an = cn − cn−1 for n ≥ 2, b1 = 1, andbn = c2
n−1 for n ≥ 2.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
A Nifty Identity
If 1c1− 1
c2+ 1
c3− · · · =
∑∞n=1
(−1)n−1
cnconverges then so does the
fraction1
c1 +c2
1
c2 − c1 +c2
2
c3 − c2 +c2
3
c4 − c3 +.. .
and they are
equivalent, i.e. they are equal at every convergent.Here a0 = 0, a1 = c1, an = cn − cn−1 for n ≥ 2, b1 = 1, andbn = c2
n−1 for n ≥ 2.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Denominator Series
Claim: kn =∏n
i=1 ci .
Proof: We proceed by a strong induction. For n = 1, k1 = a1 = c1,assuming it holds up to n = m for m ≥ 1
km+1 = am+1km + bm+1km−1
= (cm+1 − cm)m∏i=1
ci + c2m
m−1∏i=1
ci
= (cm+1 − cm)m∏i=1
ci + cm
m∏i=1
ci
=m+1∏i=1
ci . So the induction holds. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Denominator Series
Claim: kn =∏n
i=1 ci .Proof: We proceed by a strong induction. For n = 1, k1 = a1 = c1,assuming it holds up to n = m for m ≥ 1
km+1 = am+1km + bm+1km−1
= (cm+1 − cm)m∏i=1
ci + c2m
m−1∏i=1
ci
= (cm+1 − cm)m∏i=1
ci + cm
m∏i=1
ci
=m+1∏i=1
ci . So the induction holds. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Denominator Series
Claim: kn =∏n
i=1 ci .Proof: We proceed by a strong induction. For n = 1, k1 = a1 = c1,assuming it holds up to n = m for m ≥ 1
km+1 = am+1km + bm+1km−1
= (cm+1 − cm)m∏i=1
ci + c2m
m−1∏i=1
ci
= (cm+1 − cm)m∏i=1
ci + cm
m∏i=1
ci
=m+1∏i=1
ci . So the induction holds. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Denominator Series
Claim: kn =∏n
i=1 ci .Proof: We proceed by a strong induction. For n = 1, k1 = a1 = c1,assuming it holds up to n = m for m ≥ 1
km+1 = am+1km + bm+1km−1
= (cm+1 − cm)m∏i=1
ci + c2m
m−1∏i=1
ci
= (cm+1 − cm)m∏i=1
ci + cm
m∏i=1
ci
=m+1∏i=1
ci . So the induction holds. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Denominator Series
Claim: kn =∏n
i=1 ci .Proof: We proceed by a strong induction. For n = 1, k1 = a1 = c1,assuming it holds up to n = m for m ≥ 1
km+1 = am+1km + bm+1km−1
= (cm+1 − cm)m∏i=1
ci + c2m
m−1∏i=1
ci
= (cm+1 − cm)m∏i=1
ci + cm
m∏i=1
ci
=m+1∏i=1
ci . So the induction holds. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Denominator Series
Claim: kn =∏n
i=1 ci .Proof: We proceed by a strong induction. For n = 1, k1 = a1 = c1,assuming it holds up to n = m for m ≥ 1
km+1 = am+1km + bm+1km−1
= (cm+1 − cm)m∏i=1
ci + c2m
m−1∏i=1
ci
= (cm+1 − cm)m∏i=1
ci + cm
m∏i=1
ci
=m+1∏i=1
ci . So the induction holds. �
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Convergence
Claim:
limn→∞
hn
kn=∞∑n=1
(−1)n−1
cn
Proof: We have Dn = (−1)n∏n
i=1 bi = (−1)n∏n−1
i=1 c2i . Given
what we just proved about kn:
Dn
knkn−1=
(−1)n∏n−1
i=1 c2i∏n
i=1 ci ·∏n−1
i=1 ci=
(−1)n
cn
⇒ limn→∞
hn
kn= 0−
∞∑n=1
Dn
knkn−1since a0 = 0
= −∞∑n=1
(−1)n
cn=∞∑n=1
(−1)n−1
cn�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Convergence
Claim:
limn→∞
hn
kn=∞∑n=1
(−1)n−1
cn
Proof: We have Dn = (−1)n∏n
i=1 bi = (−1)n∏n−1
i=1 c2i . Given
what we just proved about kn:
Dn
knkn−1=
(−1)n∏n−1
i=1 c2i∏n
i=1 ci ·∏n−1
i=1 ci=
(−1)n
cn
⇒ limn→∞
hn
kn= 0−
∞∑n=1
Dn
knkn−1since a0 = 0
= −∞∑n=1
(−1)n
cn=∞∑n=1
(−1)n−1
cn�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Convergence
Claim:
limn→∞
hn
kn=∞∑n=1
(−1)n−1
cn
Proof: We have Dn = (−1)n∏n
i=1 bi = (−1)n∏n−1
i=1 c2i . Given
what we just proved about kn:
Dn
knkn−1=
(−1)n∏n−1
i=1 c2i∏n
i=1 ci ·∏n−1
i=1 ci=
(−1)n
cn
⇒ limn→∞
hn
kn= 0−
∞∑n=1
Dn
knkn−1since a0 = 0
= −∞∑n=1
(−1)n
cn=∞∑n=1
(−1)n−1
cn�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Convergence
Claim:
limn→∞
hn
kn=∞∑n=1
(−1)n−1
cn
Proof: We have Dn = (−1)n∏n
i=1 bi = (−1)n∏n−1
i=1 c2i . Given
what we just proved about kn:
Dn
knkn−1=
(−1)n∏n−1
i=1 c2i∏n
i=1 ci ·∏n−1
i=1 ci=
(−1)n
cn
⇒ limn→∞
hn
kn= 0−
∞∑n=1
Dn
knkn−1since a0 = 0
= −∞∑n=1
(−1)n
cn=∞∑n=1
(−1)n−1
cn�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Convergence
Claim:
limn→∞
hn
kn=∞∑n=1
(−1)n−1
cn
Proof: We have Dn = (−1)n∏n
i=1 bi = (−1)n∏n−1
i=1 c2i . Given
what we just proved about kn:
Dn
knkn−1=
(−1)n∏n−1
i=1 c2i∏n
i=1 ci ·∏n−1
i=1 ci=
(−1)n
cn
⇒ limn→∞
hn
kn= 0−
∞∑n=1
Dn
knkn−1since a0 = 0
= −∞∑n=1
(−1)n
cn=∞∑n=1
(−1)n−1
cn�
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
One final time in the actual talk, I displayed “Hey Dan, go back tothe chalkboard please.” and just talked to them. I have included it
here since there is no chalkboard.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
We can use this identity in a surprising way to find the result ofthis talk. The paper “An Elegant Continued Fraction for π” byL.J. Lange credits one (presumably Douglas) D. Bowman for thisapproach. Setting cn = 2n(2n + 1)(2n + 2) we havea1 = c1 = 2 · 3 · 4 = 6(2 · 1)2 and for n ≥ 2,an = 2n(2n + 1)(2n + 2)− (2n − 2)(2n − 1)2n = 24n2 = 6(2n)2.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
In any general continued fraction, the stepbn
an +bn+1
. . .
can be
transformed tocbn
can +cbn+1
. . .
for any c ∈ R× without changing the
value of the expression.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
For n + 1 ≥ 2 we see the fractionbn
an +bn+1
. . .
=
(2n − 2)2(2n − 1)2(2n)2
6 · (2n)2 +(2n)2(2n + 1)2(2n + 2)2
. . .
=(2n − 2)2(2n − 1)2
6 +(2n + 1)2(2n + 2)2
. . .
.
So since bn+1 occurs inbn
an +bn+1
. . .
andbn+1
an+1 +bn+2
. . .
, we cancel
(2n)2 in the first and (2n + 2)2 in the second. After this reduction,each an is simply 6 and each bn+1 becomes (2n + 1)2 for n ≥ 2.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
Since b1 = 1 we are left to deal only with b2. Our fraction hasbecome:
1
6 · 22 +223242
6 · 42 +425262
6 · 62 +.. .
=1
6 · 4 +4 · 32
6 +52
6 +. . .
.
And from our identity, this fraction is equal to
∞∑n=1
(−1)n−1
cn=∞∑n=1
(−1)n−1
2n(2n + 1)(2n + 2)=π − 3
4.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
Since b1 = 1 we are left to deal only with b2. Our fraction hasbecome:
1
6 · 22 +223242
6 · 42 +425262
6 · 62 +.. .
=1
6 · 4 +4 · 32
6 +52
6 +. . .
.
And from our identity, this fraction is equal to
∞∑n=1
(−1)n−1
cn=∞∑n=1
(−1)n−1
2n(2n + 1)(2n + 2)=π − 3
4.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
This is the point where I returned from the chalkboard back to theslides.
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
π − 3
4=
1
6 · 4 +4 · 32
6 +52
6 +. . .
=1
4
1
6 +32
6 +52
6 +. . .
⇐⇒ π = 3 +1
6 +32
6 +52
6 +. . .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
π − 3
4=
1
6 · 4 +4 · 32
6 +52
6 +. . .
=1
4
1
6 +32
6 +52
6 +. . .
⇐⇒ π = 3 +1
6 +32
6 +52
6 +. . .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
A Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Plug and Play
π − 3
4=
1
6 · 4 +4 · 32
6 +52
6 +. . .
=1
4
1
6 +32
6 +52
6 +. . .
⇐⇒ π = 3 +1
6 +32
6 +52
6 +. . .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
4π
and 12π2
Thank You
Analysis and ArithmeticA Nifty IdentityDenominator Series (kn)ConvergencePlug and PlayPlug and Play
Parting Words4π and 12
π2
Thank You
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
4π
and 12π2
Thank You
4π and 12
π2
4
π= 1 +
1
2 +9
2 +25
2 +49
2 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
4π
and 12π2
Thank You
4π and 12
π2
4
π= 1 +
1
3 +4
5 +9
7 +16
9 +.. .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
4π
and 12π2
Thank You
4π and 12
π2
12
π2= 1 +
1
3 +16
5 +81
7 +256
9 +. . .
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series
Analysis and ArithmeticParting Words
4π
and 12π2
Thank You
Thank You
Thank You for having me!
Daniel J. Hermes Life of π: Continued Fractions and Infinite Series