On (Signless) Laplacian eigenvalues of graphs · On (Signless) Laplacian eigenvalues of graphs...
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On (Signless) Laplacian eigenvalues of graphs
Kinkar Chandra Das
Department of Mathematics,Sungkyunkwan University, Rep. of Korea
Spectra of graphs and applications 2016,in honor of Prof. Dragos Cvetkovic for his 75th birthday,Serbian Academy of Sciences and Arts, Belgrade, Serbia.
May 20, 2016 (Joint work with M. Liu)
( )
Adjacency and Signless Laplacian Matrix
Conjecture 1 [1,2]:
Let G be a connected graph of order n > 3. Then
q1 − 2λ1 ≤ n− 2√n− 1
with equality holding if and only if G ∼= K1, n−1.
Adjacency and Signless Laplacian Matrix
Conjecture 1 [1,2]:
Let G be a connected graph of order n > 3. Then
q1 − 2λ1 ≤ n− 2√n− 1
with equality holding if and only if G ∼= K1, n−1.
Conjecture 2 [1,2]:
Let G be a connected graph n > 3,
1−√n − 1 ≤ q2 − λ1 ≤ n − 2−
√2 (n − 2)
with equality holding iff G ∼= K1, n−1 (lower) and G ∼= K2, n−2 (upper).
[1] M. Aouchiche, P. Hansen, A survey of automated conjectures...., Linear Algebra Appl. 432 (2010) 2293-2322.
[2] D.Cvetkovic, P. Rowlinson, S. K. Simic, Eigenvalue bounds for the signless Laplacian, Publ. Inst. Math.
(Beogr ) (N S ) 81 (95) (2007) 11–27
Relation between q1 and λ1
Theorem [3]:
Let G be a connected graph of order n > 4. Then
q2 − λ1 ≥ 1−√n − 1
with equality holding if and only if G ∼= K1, n−1 or G ∼= K5.
[3] K. C. Das, Proof of conjecture involving the second largest signless Laplacian eigenvalue and the index of
graphs, Linear Algebra Appl. 435 (2011) 2420–2424.
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
Let x = (x1, x2, . . . , xn)T be an unit eigenvector corresponding eigenvalue
q1 of Q(G ). Then
q1 =∑
vi vj∈E(G)
(xi + xj)2
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
Let x = (x1, x2, . . . , xn)T be an unit eigenvector corresponding eigenvalue
q1 of Q(G ). Then
q1 =∑
vi vj∈E(G)
(xi + xj)2 =
∑vi vj∈E(G)
(xi − xj)2 + 4
∑vi vj∈E(G)
xixj .
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
Let x = (x1, x2, . . . , xn)T be an unit eigenvector corresponding eigenvalue
q1 of Q(G ). Then
q1 =∑
vi vj∈E(G)
(xi + xj)2 =
∑vi vj∈E(G)
(xi − xj)2 + 4
∑vi vj∈E(G)
xixj .
Rayleigh-Ritz theorem,
λ1 ≥ 2∑
vi vj∈E(G)
xixj .
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
Let x = (x1, x2, . . . , xn)T be an unit eigenvector corresponding eigenvalue
q1 of Q(G ). Then
q1 =∑
vi vj∈E(G)
(xi + xj)2 =
∑vi vj∈E(G)
(xi − xj)2 + 4
∑vi vj∈E(G)
xixj .
Rayleigh-Ritz theorem,
λ1 ≥ 2∑
vi vj∈E(G)
xixj .
Hence
q1 − 2λ1 ≤n∑
i=1
di x2i − λ1
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
Let x = (x1, x2, . . . , xn)T be an unit eigenvector corresponding eigenvalue
q1 of Q(G ). Then
q1 =∑
vi vj∈E(G)
(xi + xj)2 =
∑vi vj∈E(G)
(xi − xj)2 + 4
∑vi vj∈E(G)
xixj .
Rayleigh-Ritz theorem,
λ1 ≥ 2∑
vi vj∈E(G)
xixj .
Hence
q1 − 2λ1 ≤n∑
i=1
di x2i − λ1 ≤ Δ− λ1.
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
If Δ ≤ n − 2√n − 1, then the Conjecture 1 holds.
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
If Δ ≤ n − 2√n − 1, then the Conjecture 1 holds.Otherwise,
Δ > n − 2√n − 1.
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
If Δ ≤ n − 2√n − 1, then the Conjecture 1 holds.Otherwise,
Δ > n − 2√n − 1.
If m ≥ n2
[Δ− n + 2
√n − 1
], then also Conjecture 1 holds.
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
If Δ ≤ n − 2√n − 1, then the Conjecture 1 holds.Otherwise,
Δ > n − 2√n − 1.
If m ≥ n2
[Δ− n + 2
√n − 1
], then also Conjecture 1 holds.
Still we have to prove our Conjecture 1 for Δ > n − 2√n − 1 and
m < n2
[Δ− n+ 2
√n− 1
].
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
If Δ ≤ n − 2√n − 1, then the Conjecture 1 holds.Otherwise,
Δ > n − 2√n − 1.
If m ≥ n2
[Δ− n + 2
√n − 1
], then also Conjecture 1 holds.
Still we have to prove our Conjecture 1 for Δ > n − 2√n − 1 and
m < n2
[Δ− n+ 2
√n− 1
].
q1 − 2λ1 ≤∑
vi vj∈E(G)
(xi − xj)2
Adjacency and Signless Laplacian Matrix
Partial Proof of Conjecture 1:
If Δ ≤ n − 2√n − 1, then the Conjecture 1 holds.Otherwise,
Δ > n − 2√n − 1.
If m ≥ n2
[Δ− n + 2
√n − 1
], then also Conjecture 1 holds.
Still we have to prove our Conjecture 1 for Δ > n − 2√n − 1 and
m < n2
[Δ− n+ 2
√n− 1
].
q1 − 2λ1 ≤∑
vi vj∈E(G)
(xi − xj)2 ≤ m (xmax − xmin)
2 ≤ n
2
[Δ− n + 2
√n − 1
]
× (xmax − xmin)2 .
Conjecture 1 for tree
Proof of Conjecture 1 for tree:
Let mi be the average degree of the adjacent vertices of vertex vi .
Conjecture 1 for tree
Proof of Conjecture 1 for tree:
Let mi be the average degree of the adjacent vertices of vertex vi .We have
q1 ≤ max1≤i≤n
(di +mi )
Conjecture 1 for tree
Proof of Conjecture 1 for tree:
Let mi be the average degree of the adjacent vertices of vertex vi .We have
q1 ≤ max1≤i≤n
(di +mi ) = dk +mk , (say).
Conjecture 1 for tree
Proof of Conjecture 1 for tree:
Let mi be the average degree of the adjacent vertices of vertex vi .We have
q1 ≤ max1≤i≤n
(di +mi ) = dk +mk , (say).
Moreover, for Tree
λ1 ≥ max1≤i≤n
√di +mi − 1
Conjecture 1 for tree
Proof of Conjecture 1 for tree:
Let mi be the average degree of the adjacent vertices of vertex vi .We have
q1 ≤ max1≤i≤n
(di +mi ) = dk +mk , (say).
Moreover, for Tree
λ1 ≥ max1≤i≤n
√di +mi − 1 ≥
√dk +mk − 1.
Conjecture 1 for tree
Proof of Conjecture 1 for tree:
Let mi be the average degree of the adjacent vertices of vertex vi .We have
q1 ≤ max1≤i≤n
(di +mi ) = dk +mk , (say).
Moreover, for Tree
λ1 ≥ max1≤i≤n
√di +mi − 1 ≥
√dk +mk − 1.
Then
q1 − 2λ1 ≤ (√
dk +mk − 1− 1)2
Conjecture 1 for tree
Proof of Conjecture 1 for tree:
Let mi be the average degree of the adjacent vertices of vertex vi .We have
q1 ≤ max1≤i≤n
(di +mi ) = dk +mk , (say).
Moreover, for Tree
λ1 ≥ max1≤i≤n
√di +mi − 1 ≥
√dk +mk − 1.
Then
q1 − 2λ1 ≤ (√
dk +mk − 1− 1)2 ≤ (√n− 1− 1)2 = n − 2
√n − 1.
Algebraic Connectivity
Conjecture 3 [4,5,6]:
a(G )/δ(G ) is minimum for graph composed of 2 triangles linked with apath.
[4] M. Aouchiche, Comparaison Automatisee d’Invariants en Theorie des Graphes, Ph.D. Thesis, Ecole
Polytechnique de Montreal, February 2006.
[5] M. Aouchiche, G. Caporossi, P. Hansen, Variable neighborhood search for extremal graphs. 20. Automated
comparison of graph invariants, MATCH Commun. Math. Comput. Chem. 58 (2007) 365–384.
[6] M. Aouchiche, P. Hansen, A survey of automated conjectures in spectral graph theory, Linear Algebra Appl. 432
(2010) 2293–2322.
Notation:
A path with n vertices is denoted by Pn.
Notation:
A path with n vertices is denoted by Pn.
The near path Qn is the tree on n vertices obtained from a path Pn−1 :v1v2 · · · vn−2vn−1 by attaching a new pendant edge vn−2vn at vn−2.
Notation:
A path with n vertices is denoted by Pn.
The near path Qn is the tree on n vertices obtained from a path Pn−1 :v1v2 · · · vn−2vn−1 by attaching a new pendant edge vn−2vn at vn−2.
Let Wn be the tree on n vertices obtained from a path Pn−2 :v2v3 · · · vn−2vn−1 by attaching a new pendant edge vn−2vn at vn−2 andanother new pendant edge v1v3 at v3, respectively.
Notation:
A path with n vertices is denoted by Pn.
The near path Qn is the tree on n vertices obtained from a path Pn−1 :v1v2 · · · vn−2vn−1 by attaching a new pendant edge vn−2vn at vn−2.
Let Wn be the tree on n vertices obtained from a path Pn−2 :v2v3 · · · vn−2vn−1 by attaching a new pendant edge vn−2vn at vn−2 andanother new pendant edge v1v3 at v3, respectively.
Let Q ′n = Qn + vn−1vn, W
′n = Wn + v1v2 and W ′′
n = Wn + v1v2 + vn−1vn.
Notation:
A path with n vertices is denoted by Pn.
The near path Qn is the tree on n vertices obtained from a path Pn−1 :v1v2 · · · vn−2vn−1 by attaching a new pendant edge vn−2vn at vn−2.
Let Wn be the tree on n vertices obtained from a path Pn−2 :v2v3 · · · vn−2vn−1 by attaching a new pendant edge vn−2vn at vn−2 andanother new pendant edge v1v3 at v3, respectively.
Let Q ′n = Qn + vn−1vn, W
′n = Wn + v1v2 and W ′′
n = Wn + v1v2 + vn−1vn.
Let Zn be the tree on n vertices obtained from a path Pn−1 :v1v2 · · · vn−2vn−1 by attaching a new pendant edge vn−3vn at vn−3.
Inequality
Lemma 1:
For x > 0,
x > sin x > x − x3
6.
Inequality
Lemma 1:
For x > 0,
x > sin x > x − x3
6.
Lemma 2:
For any positive integer n > 3,
sin( π
2 n
)>
1√2sin
(π
2 (n − 1)
).
Inequality
Proof:
For x > 0,
sin( π
2n
)>
π
2n− π3
48 n3>
π
2√2 (n − 1)
>1√2sin
(π
2 (n − 1)
).
Laplacian Eigenvalues
Lemma 3:
The Laplacian eigenvalues of path Pn are
2 + 2 cos(π i
n
), i = 1, 2, . . . , n − 1 and 0.
Algebraic Connectivity
Definition:
We define
Ad(G ) =a(G )
δ(G ).
Algebraic Connectivity
Definition:
We define
Ad(G ) =a(G )
δ(G ).
Algebraic Connectivity
Definition:
We define
Ad(G ) =a(G )
δ(G ).
Conjecture 3:
Let G be a connected graph of order n > 3 and minimum degree δ. Then
Ad(G ) ≥ Ad(W ′′n ) (1)
with equality holding if and only if G ∼= W ′′n .
Algebraic Connectivity
Theorem 1:
Let G be a connected graph of order n > 3 and minimum degree δ. If δ ≤ 2or δ ≥ n/2, then
Ad(G ) ≥ Ad(W ′′n ) (2)
with equality holding if and only if G ∼= W ′′n .
Algebraic Connectivity
Theorem 1:
Let G be a connected graph of order n > 3 and minimum degree δ. If δ ≤ 2or δ ≥ n/2, then
Ad(G ) ≥ Ad(W ′′n ) (2)
with equality holding if and only if G ∼= W ′′n .
Algebraic Connectivity
Theorem 1:
Let G be a connected graph of order n > 3 and minimum degree δ. If δ ≤ 2or δ ≥ n/2, then
Ad(G ) ≥ Ad(W ′′n ) (2)
with equality holding if and only if G ∼= W ′′n .
Proof:
For 4 ≤ n ≤ 8, one can easily check the result by Sage. Otherwise, n ≥ 9.
Lemma 4 [7]:
Let G be a connected graph of order n ≥ 9 andG /∈ {Pn, Qn, Q
′n, Wn, W
′n, W
′′n }. Then we have
a(Pn) < a(Qn) = a(Q ′n) < a(Wn) = a(W ′
n) = a(W ′′n ) < a(G ), a(Wn) < a(Zn
Lemma 4 [7]:
Let G be a connected graph of order n ≥ 9 andG /∈ {Pn, Qn, Q
′n, Wn, W
′n, W
′′n }. Then we have
a(Pn) < a(Qn) = a(Q ′n) < a(Wn) = a(W ′
n) = a(W ′′n ) < a(G ), a(Wn) < a(Zn
Lemma 5 [8]:
If v is a pendent vertex, then a(G ) ≤ a(G − v).
[7] J.-Y. Shao, J.-M. Guo, H.-Y. Shan, The ordering of trees and connected graphs by algebraic connectivity, Linear
Algebra Appl. 428 (2008) 1421–1438.
[8] J. X. Li, J.-M. Guo, W. C. Shiu, The Smallest Values of Algebraic Connectivity for Trees, Acta Mathematica
Sinica, English Series 28 (10) (2012) 2021–2032.
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G )
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n )
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn)
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn) = Ad(Q ′n)
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn) = Ad(Q ′n) < Ad(Wn)
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn) = Ad(Q ′n) < Ad(Wn) = Ad(W ′
n).
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn) = Ad(Q ′n) < Ad(Wn) = Ad(W ′
n).
Now we have to show that
Ad(Pn) > Ad(W ′′n )
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn) = Ad(Q ′n) < Ad(Wn) = Ad(W ′
n).
Now we have to show that
Ad(Pn) > Ad(W ′′n ), i.e., a(Wn) = a(W ′′
n )
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn) = Ad(Q ′n) < Ad(Wn) = Ad(W ′
n).
Now we have to show that
Ad(Pn) > Ad(W ′′n ), i.e., a(Wn) = a(W ′′
n ) < 2 a(Pn)
Proof:
Case (i): δ = 1.
If G /∈ {Pn, Qn, Q′n, Wn, W
′n}, then by Lemma 4, we get
Ad(G ) = a(G ) > a(W ′′n ) >
a(W ′′n )
2= Ad(a(W ′′
n )).
Moreover, we have
Ad(Pn) < Ad(Qn) = Ad(Q ′n) < Ad(Wn) = Ad(W ′
n).
Now we have to show that
Ad(Pn) > Ad(W ′′n ), i.e., a(Wn) = a(W ′′
n ) < 2 a(Pn) = 2(2− 2 cos
(πn
))
Proof:
Case (i): δ = 1.
By Lemma 1.2, we have
2 sin2( π
2n
)> sin2
(π
2(n − 1)
)
Proof:
Case (i): δ = 1.
By Lemma 1.2, we have
2 sin2( π
2n
)> sin2
(π
2(n − 1)
), i.e., 1− 2 cos
(πn
)+ cos
(π
n − 1
)> 0
Proof:
Case (i): δ = 1.
By Lemma 1.2, we have
2 sin2( π
2n
)> sin2
(π
2(n − 1)
), i.e., 1− 2 cos
(πn
)+ cos
(π
n − 1
)> 0
Using the above result with some Lemmas 1.3, 1.4 and 1.5, we have
a(Wn) < a(Zn)
Proof:
Case (i): δ = 1.
By Lemma 1.2, we have
2 sin2( π
2n
)> sin2
(π
2(n − 1)
), i.e., 1− 2 cos
(πn
)+ cos
(π
n − 1
)> 0
Using the above result with some Lemmas 1.3, 1.4 and 1.5, we have
a(Wn) < a(Zn) ≤ a(Pn−1)
Proof:
Case (i): δ = 1.
By Lemma 1.2, we have
2 sin2( π
2n
)> sin2
(π
2(n − 1)
), i.e., 1− 2 cos
(πn
)+ cos
(π
n − 1
)> 0
Using the above result with some Lemmas 1.3, 1.4 and 1.5, we have
a(Wn) < a(Zn) ≤ a(Pn−1) = 2− 2 cos
(π
n − 1
)
Proof:
Case (i): δ = 1.
By Lemma 1.2, we have
2 sin2( π
2n
)> sin2
(π
2(n − 1)
), i.e., 1− 2 cos
(πn
)+ cos
(π
n − 1
)> 0
Using the above result with some Lemmas 1.3, 1.4 and 1.5, we have
a(Wn) < a(Zn) ≤ a(Pn−1) = 2− 2 cos
(π
n − 1
)< 2
(2− 2 cos
(πn
))
Proof:
Case (i): δ = 1.
By Lemma 1.2, we have
2 sin2( π
2n
)> sin2
(π
2(n − 1)
), i.e., 1− 2 cos
(πn
)+ cos
(π
n − 1
)> 0
Using the above result with some Lemmas 1.3, 1.4 and 1.5, we have
a(Wn) < a(Zn) ≤ a(Pn−1) = 2− 2 cos
(π
n − 1
)< 2
(2− 2 cos
(πn
))
= 2 a(Pn).
Proof:
Case (ii): δ = 2.
Then by Lemma 1.4,
Ad(G ) =a(G )
2≥ a(W ′′
n )
2= Ad(W ′′
n )
with equality holding if and only if G ∼= W ′′n .
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2< 2− 2 cos
(πn
)
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2< 2− 2 cos
(πn
)= 4 sin2
( π
2n
)
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2< 2− 2 cos
(πn
)= 4 sin2
( π
2n
)<
π2
n2.
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2< 2− 2 cos
(πn
)= 4 sin2
( π
2n
)<
π2
n2.
Since n ≥ 5, one can easily see that
n
(1− π2
n2
)> n − 2 .
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2< 2− 2 cos
(πn
)= 4 sin2
( π
2n
)<
π2
n2.
Since n ≥ 5, one can easily see that
n
(1− π2
n2
)> n − 2 .
Thus we have
δ
(1− π2
n2
)≥ n
2
(1− π2
n2
)
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2< 2− 2 cos
(πn
)= 4 sin2
( π
2n
)<
π2
n2.
Since n ≥ 5, one can easily see that
n
(1− π2
n2
)> n − 2 .
Thus we have
δ
(1− π2
n2
)≥ n
2
(1− π2
n2
)>
n − 2
2
Proof:
Case (iii): δ ≥ n/2.
By Case (i) and Lemma 1.1, we have
Ad(W ′′n ) =
a(W ′′n )
2< 2− 2 cos
(πn
)= 4 sin2
( π
2n
)<
π2
n2.
Since n ≥ 5, one can easily see that
n
(1− π2
n2
)> n − 2 .
Thus we have
δ
(1− π2
n2
)≥ n
2
(1− π2
n2
)>
n − 2
2, i.e.,
π2
n2< 1− n − 2
2 δ.
Case (iii): δ ≥ n/2
Lemma 1.6 [9]:
Let G be a connected graph of order n, minimum degree δ(G )and algebraic connectivity a(G ). Then
a(G )− δ(G ) ≥
⎧⎪⎨⎪⎩
−n − 8 +√n2 + 8n − 16
4if n is even
−n − 3
2if n is odd
Moreover, the equality holds if and only if G ∼= Kn/2, n/2\{e}when n is even (e is any edge), and G ∼= K(n−1)/2, (n−1)/2 ∨ K1
when n is odd.
[9] K. C. Das, Proof of conjectures involving algebraic connectivity of graphs, Linear Algebra Appl. 438 (2013)
3291–3302.
Proof:
Case (iii): δ ≥ n/2.
From Lemma 1.6, we have
−n− 2
2≤ −n− 8 +
√n2 + 8n − 16
4≤ −n− 3
2≤ a(G )− δ.
Proof:
Case (iii): δ ≥ n/2.
From Lemma 1.6, we have
−n− 2
2≤ −n− 8 +
√n2 + 8n − 16
4≤ −n− 3
2≤ a(G )− δ.
Therefore
Ad(G ) =a(G )
δ
Proof:
Case (iii): δ ≥ n/2.
From Lemma 1.6, we have
−n− 2
2≤ −n− 8 +
√n2 + 8n − 16
4≤ −n− 3
2≤ a(G )− δ.
Therefore
Ad(G ) =a(G )
δ≥ 1− n − 2
2 δ
Proof:
Case (iii): δ ≥ n/2.
From Lemma 1.6, we have
−n− 2
2≤ −n− 8 +
√n2 + 8n − 16
4≤ −n− 3
2≤ a(G )− δ.
Therefore
Ad(G ) =a(G )
δ≥ 1− n − 2
2 δ>
π2
n2
Proof:
Case (iii): δ ≥ n/2.
From Lemma 1.6, we have
−n− 2
2≤ −n− 8 +
√n2 + 8n − 16
4≤ −n− 3
2≤ a(G )− δ.
Therefore
Ad(G ) =a(G )
δ≥ 1− n − 2
2 δ>
π2
n2> Ad(W ′′
n ).
Proof:
Remark:
For 3 ≤ δ < n/2, Conjecture 3 is still open.
THANK YOU for attention.