# Category Archives: Uncategorized

## Sum of largest two eigenvalues of a simple graph.3

July 2, 2008

The following graphs give counter-examples to Gernert’s conjecture $\lambda_1+\lambda_2 \leq n$ for graphs $\Gamma$ with $n$ vertices, for $n = 34, 36, 37$ though the examples do not have a connected complementary graph $\Gamma^{\circ}$.

Together with Nikiforov’s 2006 paper:

Nikiforov, V. (2006) Linear combinations of graph eigenvalues. Electronic Journal of Linear Algebra, 15, 329-336. (nikiforov-sums_of_graph_eigenvalues)

and the recent (June 13, 2008 ) paper of Javad Ebrahimi, Bojan Mohar, Vladimir Nikiforov and Azhvan Sheikh Ahmady (eigenvalue_sum_extended), it’s beginning to look more likely that for all $n \geq 21$ there is a graph $\Gamma$ on $n$ vertices with $\lambda_1 + \lambda > n$. As Bojan Mohar remarks, following the construction in his recent paper, for $n \equiv 0 \textrm{ (mod 7)}$ and sufficiently large, there is such a $\Gamma$ with $\Gamma^{\circ}$ conected.

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The graph with 37 vertices and 657 edges, and adjacency matrix that has zeros in the following places:

(1, 1), (2, 2), (2, 14), (2, 17), (2, 19), (2, 26), (2, 34), (2, 37), (3, 3), (4, 4), (5, 5), (6, 6), (6, 8), (7, 7), (8, 6), (8, 8), (9, 9), (9, 34), (10, 10), (11, 11), (12, 12), (12, 14), (13, 13), (14, 2), (14, 12), (14, 14), (15, 15), (16, 16), (17, 2), (17, 17), (18, 18), (19, 2), (19, 19), (20, 20), (21, 21), (22, 22), (23, 23), (24, 24), (25, 25), (26, 2), (26, 26), (27, 27), (28, 28), (29, 29), (30, 30), (31, 31), (32, 32), (33, 33), (34, 2), (34, 9), (34, 34), (35, 35), (36, 36), (37, 2), (37, 37)

has sum of the two largest eigenvalues $\lambda_1 +\lambda_2 \approx 37.0176 > 37$.

It has characteristic polynomial $-x^2(1+x)^28(2+x)(128+642x+344x^2-226x^3-191x^4-30x^5+x^6)$

numerical spectrum (-3.08297, -2., -1.92919, -1.77378, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.231694, -2.13825*10^-17, 7.84933*10^-17, 1.47326, 35.5444),

degree sequence (36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 35, 35, 35, 35, 35, 35, 35, 35, 34, 34, 30)

and energy $\approx 74.0353$.

The complement is disconnected:

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The graph with 36 vertices and 622 edges, and adjacency matrix that has zeros in the following places:

(1,1), (2,2), (3,3), (3,16), (4,4), (5,5), (6,6), (7,7), (8,8), (9,9), (9,16), (9,33), (10,10), (11,11), (12,12), (13,13), (14,14), (15,15), (16,3), (16,9), (16,16), (16,19), (16,24), (16,35), (17,17), (18,18), (19,16), (19,19), (20,20), (21,21), (22,22), (23,23), (23,36), (24,16), (24,24), (24,33), (25,25), (26,26), (27,27), (28,28), (29,29), (30,30), (31,31), (32,32), (33,9), (33,24), (33,33), (34,34), (35,16), (35,35), (36,23), (36,36)

has sum of the two largest eigenvalues $\lambda_1 +\lambda_2 \approx 36.0019 > 36$.

It has characteristic polynomial $x(1+x)^29(2+x)(4+290x+31x^2-125x^3-31x^4+x^5)$

numerical spectrum (-3.06372,-2.,-1.92434,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,-1.,1.42018,34.5817),

degree sequence (35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 35, 34, 34, 34, 34, 34, 33, 33, 33, 30)

and energy $\approx 57.98994$.

The complement is disconnected:

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The graph with 34 vertices and 553 edges, and adjacency matrix that has zeros in the following places:

(1,1), (2,2), (3,3), (3,6), (4,4), (5,5), (6,3), (6,6), (6,11), (6,27), (7,7), (8,8), (9,9), (10,10), (10,27), (11,6), (11,11), (11,25), (12,12), (13,13), (14,14), (15,15), (16,16), (17,17), (18,18), (19,19), (20,20), (21,21), (22,22), (23,23), (24,24), (25,11), (25,25), (25,27), (26,26), (27,6), (27,10), (27,25), (27,27), (27,29), (27,30), (28,28), (29,27), (29,29), (30,27), (30,30), (31,31), (32,32), (33,33), (34,34)

has sum of the two largest eigenvalues $\lambda_1 +\lambda_2 \approx 34.063309767691338026 > 34$.

It has characteristic polynomial $(1+x)^27(-21+133x+622x^2+328x^3-209x^4-175x^5-27x^6+x^7)$

numerical spectrum (-3.03079, -2.13762, -1.60305, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.396219, 0.10436, 1.5018, 32.5615),

degree sequence (33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 32, 32, 32, 32, 31, 31, 30, 28 )

and energy $\approx 68.3353$.

The complement is disconnected:

## Sum of largest two eigenvalues of a simple graph.2

Below are several other examples of non-isomorphic graphs with 40 vertices, 770 edges, and sum of the two largest eigenvalues > 40. None of these has a connected complement.

1. The graph $\Gamma_2(40,770)$ with $v=40$ vertices and $e = 770$ edges, and adjacency matrix:

has $\lambda_1+\lambda_2 \approx 40.0086 > v=40$.

The characteristic polynomial is $x(1+x)^31 (-68-393x+1131x^2 +1681x^3 +101x^4-622x^5 -274x^6-31x^7+x^8)$ and the sorted numerical spectrum is (-3.10756, -2.32462, -1.93923, -1.85196, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.132379, -4.09544*10^-16, 0.347119, 1.48047, 38.5282).

The graph has (sorted) degree sequence (39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 38, 38, 38, 38, 38, 38, 38, 38, 37, 37, 37, 33).

The energy is $E(\Gamma_2(40,770)) \approx 80.7115$.

The graph $\Gamma_2(40,770)$ is, therefore, not isomorphic with $\Gamma_1(40,772)$

2. The graph$\Gamma_3(40,770)$ with $v=40$ vertices and $e=770$ edges with the following adjacency matrix:

has $\lambda_1+\lambda_2 \approx 40.0284 > 40$.

The characteristic polynomial is $x^2(1+x)^29(2+x)(-50+271x +1577x^2+1506x^3 -74x^4 -649x^5-273x^6-31x^7+x^8)$ and the sorted numerical spectrum is (-3.13442, -2.13787, -2., -1.86594, -1.60276, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.396136, -8.94552*10^-17, 1.54841*10^-16, 0.108693, 1.50412, 38.5243).

The graph has (sorted) degree sequence (39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 38, 38, 38, 38, 38, 38, 38, 38, 37, 37, 37, 33).

The energy is $E(\Gamma_3(40,770)) \approx 80.2743$.

The graph $\Gamma_3(40,770)$ is, therefore, not isomorphic with $\Gamma_1(40,772)$ or with $\Gamma_2(40,772)$.

3. The graph$\Gamma_4(40,770)$ with $v=40$ vertices and $e=770$ edges with the following adjacency matrix:

has $\lambda_1+\lambda_2 \approx 40.0473 > 40$.

The characteristic polynomial is $latex(1+x)^31(33-137x-454x^2+1202x^3 +1783x^4+139x^5 -618x^6-274x^7-31x^8+x^9)$ and the sorted numerical spectrum is (-3.04901, -2.30734, -2.20329, -1.65743, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.346656, 0.199239, 0.317189, 1.52195, 38.5253).

The graph has (sorted) degree sequence (39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 38, 38, 38, 38, 38, 38, 38, 38, 37, 37, 37, 33).

The energy is $E(\Gamma_3(40,770)) \approx 81.1274$.

The graph $\Gamma_4(40,770)$ is, therefore, not isomorphic with $\Gamma_1(40,772), \Gamma_2(40,770)$ or $\Gamma_3(40,772)$.

4. The graph$\Gamma_5(40,770)$ with $v=40$ vertices and $e=770$ edges with the following adjacency matrix:

has $\lambda_1+\lambda_2 \approx 40.0716> 40$.

The characteristic polynomial is $x(1+x)^31(-132-296x+1318x^2+1776x^3+ 122x^4- 620x^5-274x^6-31x^7+x^8)$ and the sorted numerical spectrum is (-3.12149, -2.35749, -1.92974, -1.75915, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.253681, 1.79662*10^-17, 0.349958, 1.54489, 38.5267).

The graph has (sorted) degree sequence (39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 38, 38, 38, 38, 38, 37, 36, 36, 36, 35).

The energy is $E(\Gamma_3(40,770)) \approx 80.8431$.

The graph $\Gamma_5(40,770)$ is, therefore, not isomorphic with $\Gamma_1(40,772), \Gamma_2(40,770)$, $\Gamma_3(40,772)$ or $\Gamma_4(40,772)$

An example of a graph with $v = 39, e = 731$ and $\lambda_1+\lambda_2 \approx 39.0523 > 39$ is $\Gamma_1(39,731)$ with the following adjacency matrix and 3-dimensional representation:

The characteristic polynomial is $-x(1+x)^30(2+x)(-4-164x+674x^2+523x^3-196x^4 -202x^5-32x^6+x^7)$ and the sorted numerical spectrum is (-3.08933, -2.24518, -2., -1.92719, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.0223697, -4.2055*10^-16, 0.231765, 1.53858, 37.5137).

The graph has (sorted) degree sequence (38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38, 38 ,38 ,38 ,38, 38, 38, 38, 38,38,38,38,38, 38, 38, 38, 38, 38, 37, 37, 37, 37, 37, 37, 37, 36, 35, 34, 34).

The energy is $E(\Gamma_1(39,731) \approx 78.5681$.

A 3 dimensional representation of the complementray graph is:

The vetcies apparently connected at the bottom left are in fact not connected, as the following, more prosaic, view shows:

I found this example by looking for a graph with 39 vertices, and taking a cue from $\Gamma_1(40,770)$, looked for one with $e \approx \frac{1}{2}39\times 38 \times \frac{770}{780} = 731.5$.

Using the same strategy I found a graph $\Gamma_1(38,694)$ with $v= 38$ vertices, $e = 694$ edges, and $\lambda_1+\lambda_2 \approx 38.0326> 38$. Apart from the diagonal the 0 entries in the adjacency matrix are in the following positions:

(5, 19), (8, 32), (9, 32), (14, 30), (14, 32), (16, 26), (19, 5), (19, 30), (19, 32), (19, 33), (26, 16),(30, 14), (30, 19),(32, 8), (32, 9), (32, 14), (32, 19), (33, 19).

The characteristic polynomial is $x(1+x)^29(-1-3x+x^2+x^3)(48-297x- 484x^2-226x^3- 30x^4+x^5)$, the numerical spectrum (-3.06878, -2.17009, -1.92725, -1.68699, -1., -1., -1., -1., -1., -1., \-1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.311108, 2.86673*10^-16, 0.13162, 1.48119, 36.5514),the degree sequence (37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 36, 36, 36, 36, 36, 36, 35, 35, 33, 33) and the energy $\approx 76.3284$. Again, the complementary graph is disconnected:

Bojan Mohar just sent me a copy of his paper “On the sum of two largest eigenvalues of a symmetric matrix” (June 13, 2008 ) with Javad Ebrahimi, Vladimir Nikiforov and Azhvan Sheikh Ahmady.

Here’s the abstract:

D. Gernert conjectured that the sum of two largest eigenvalues of the adjacency matrix of any simple graph is at most the number of vertices of the graph. This can be proved, in particular, for all regular graphs. Gernert’s conjecture was recently disproved by one of the authors, who also provided a nontrivial upper bound for the sum of two largest eigenvalues. In this paper we improve the lower and upper bounds to near-optimal ones, and extend results from graphs to general non-negative matrices.

Bojan Mohar comments (July 1, 2008 ) that Dieter Gernert’s modified conjecture cannot hold: “the proof towards the end of the paper shows that largest $\lambda_1+ \lambda_2$ is achieved when the complement is bipartite and (most likely) has several isolated vertices. The updated conjecture seems in a good direction but it is wrong again: Take our example (same paper) with $7n$ vertices and $\tau_2 = \frac{8}{7}$ . Add one isolated vertex (or join it to the graph by one edge if you wish). This will make the complement connected but $\tau_2$ will not change much if $n$ is large.”

## Sum of largest two eigenvalues of a simple graph.1

The largest two, necessarily real, eigenvalues of a simple graph $\Gamma$ play a significant role in the energy $E(\Gamma)$ of $\Gamma$. The largest eigenvalue $\lambda_1$ is bounded below by $\frac{2e}{v}$ where $v$ is the number of vertices and $E$ the number of edges of $\Gamma$. The largest eigenvalue features in Koolen & Moulton’s proof of the upper bound $E(\Gamma) \leq \frac{2e}{v}+\sqrt{(v-1)(2e-(\frac{2e}{v})^2)}$. Some knowledge of the second largest eigenvalue $\lambda_2$ might go some way toward sharpening this upper bound for $E(\Gamma)$ in those – mostly common -cases in which this upper bound is not attained.

Dieter Gernert has proposed that the sum $\lambda_1 + \lambda_2$ of the largest two eigenvalues of a simple graph on $v$ vertices should be bounded above by $v$. As he points out on the Spectral Graph Theory page, this is certainly true for regular graphs, and has also been proved for some special classes of graphs such as planar, toroidal, completely multipartite, and triangular-free graphs.

However, the graph $\Gamma_1(40,770)$, with 40 vertices and 770 edges, whose adjacency matrix and – rather uninformative though beautiful – 3-dimensional representation appear below, has a sum of $\approx 40.0032 > 40$ for the sum of its two largest eigenvalues:

The characteristic polynomial is $x(1+x)^{31}(-132-496x+1106x^2+1716x^3+120x^4-620x^5-274x^6-31x^7+x^8)$ and the sorted numerical spectrum is (-3.02512, -2.47643, -1.93174, -1.8057, -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -1., -0.202583, -6.38638×10-17, 0.438356, 1.47645, 38.5268)

The graph has (sorted) degree sequence (39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 39, 38, 38, 38, 38, 38, 38, 38, 38, 38, 37, 35, 34).

The energy is $E(\Gamma_1(40,770)) \approx 80.8831$ which is $\approx 94.1\%$ of the Koolen-Moulten upper bound $\frac{2e}{v}+\sqrt{(v-1)(2e-(\frac{2e}{v})^2)}$ for the energy of a graph with $v=40$ vertices and $e=770$ edges.

A statistical analysis of $v-(\lambda_1+\lambda_2)$ for graphs with $13 \leq v \leq 60$ vertices and $v-1 \leq e \leq \frac{1}{2}v(v-1)-1$ edges shows a very skewed distribution with a large mean $\approx 9$ and large standard deviation $\approx 6$:

Occasionally, but apparently rarely, despite the large standard deviation, $v-(\lambda_1+\lambda_2)$ is negative, as it is for $\Gamma_1(40,770)$.

Postscript: I just found this question has been answered. A definitive paper, on the existence of graphs with $v< (\lambda_1+\lambda_2)$ is:

Nikiforov, V. (2006) Linear combinations of graph eigenvalues. Electronic Journal of Linear Algebra, 15, 329-336. (nikiforov-sums_of_graph_eigenvalues)

Nikiforov’s inequality $max(\lambda_1+\lambda_2) >\frac{29 + \sqrt(329)}{42}v -25$ for $v \geq 21$ shows that for all $v \geq 205$ there is a graph with $\lambda_1+\lambda_2 > v$ .

Given the existence of $\Gamma_1(40,770)$, what is the minimal $v$ for which there is a graph on $v$ vertices with $\lambda_1 + \lambda_2 > v$?

Given Nikiforov’s construction, Dieter Gernert has modified his question to: “Is is true that $\lambda_1+\lambda_2 \leq v$ for all connected graphs $\Gamma$ with $v$ vertices and connected complementary graph $\Gamma^{\circ}$?” Dieter conjectures this is true.