## Eigenvalues and Energy of the Cayley Graph of

## some Groups with respect to a Normal Subset

### Maryam Jalali

### −

### Rad

?Abstract

Set X = {M11, M12, M22, M23, M24, Zn, T4n, SD8n, Sz(q), G2(q), V8n},

where M11,M12,M22, M23,M24 are Mathieu groups andZn, T4n,SD8n, Sz(q),G2(q)andV8n denote the cyclic, dicyclic, semi-dihedral, Suzuki, Ree

and a group of order8npresented by

V8n=ha, b|a2n=b4=e, aba=b−1, ab−1a=bi,

respectively. In this paper, we compute all eigenvalues ofCay(G, T), where G ∈ X and T is minimal, second minimal, maximal or second maximal normal subset of G\ {e} with respect to its size. In the case that S is a minimal normal subset of G\ {e}, the summation of the absolute value of eigenvalues, energy of the Cayley graph, is evaluated.

Keywords: Simple group, Cayley graph, eigenvalue, energy.

2010 Mathematics Subject Classification: 20D08,05C50,20C15.

## 1. Introduction

SupposeΓ = (V, E)is a simple graph, whereV =V(Γ) is the set of vertices and

E=E(Γ)is the set of edges ofΓ. IfGis a finite group,S is a subset of Gsuch that S =S−1 and S ⊆G\ {e} then the Cayley graph Γ =Cay(G, S)is defined byV(Γ) = Gand E(Γ) ={{g, sg} | g ∈G, s ∈S} [4]. It is clear that a Cayley graphΓ =Cay(G, S)is connected if and only ifG=hSi.

A subsetS of a finite group Gis called normal, ifg−1Sg =S, for allg ∈G. It is called symmetric, ifS−1=S. A normal, symmetric and generating subset of

Gwith this property thate6∈S is said to be anN S of G. A minimal or second minimalN S is denoted byM N S orSM N S, respectively.

?_{Corresponding author (E-mail: jalali6834@gmail.com )}

Academic Editor: Ivan Gutman

Received 21 August 2017, Accepted 6 November 2017 DOI: 10.22052/mir.2017.101675.1082

c

The semi-dihedral groupSD8n, dicyclic groupT4n and the groupV8n have the following presentations, respectively:

SD8n = ha, b|a4n=b2=e, bab=a2n−1i,

T4n = ha, b|a2n= 1, an=b2, b−1ab=a−1i,

V8n = ha, b|a2n=b4=e, aba=b−1, ab−1a=bi.

It is easy to see the dicyclic groupT4n has order4nand the cyclic subgrouphaiof

T4nhas index 2 [13]. The groupsSD8n andV8n have order8nand their character tables computed in [11, 6], respectively.

Set X = {M11, M12, M22, M23, M24, Zn, T4n, SD8n, Sz(q), G2(q), V8n}, where

M11, M12, M22, M23, M24 are denoted Mathieu groups, Sz(q) is Suzuki group of order(q−1)(q2+ 1)and G2(q) is Ree group. The energyE(G)of a graph G is defined as the sum of the absolute values of the eigenvalues of the adjacency matrix [10]. The aim of this paper is to compute the eigenvalues of Cay(G, S), whereS is anM N S orSM N S andGis isomorphic to a group inX. In the case thatSis a minimal normal subset ofG\ {e}, the summation of the absolute value of eigenvalues, energy of the Cayley graph, is evaluated.

A subsetA ⊆G is called rational if, for every characterχ∈Irr(G),χ(A) =

P

x∈Aχ(x)is an integer. Alpering in some recent papers [1, 2], proved some very nice results on integrality of Cayley graphs. Among these results, two are very important as follows:

• if the Cayley graph of Gon a setS is an integral Cayley graph thenS is a rational set.

• for abelian groups a setS is rational if and only if the Cayley graph onS is an integral Cayley graph.

In this paper we study the integral Cayley graphs on simple group. Our calcu-lations are made by computer algebra system GAP [17]. Our notation is standard and can be taken from [5, 12, 13].

## 2. Main Results

To compute the eigenvalues of a Cayley graph Cay(G, S) with respect to a normal subsetSof groupG\ {e}, we apply a result of Zieschang [19, Theorem 1]. For the sake of completeness, we mention here this result.

Proposition 2.1. Let Cay(G, T) denote the Cayley graph of a finite group G with respect to a normal subset T of G\ {e}. Let further{χ1, . . . , χs} be the set

of all irreducible complex characters ofGand defineλj= _{χ}1
j(e)

P

t∈Tχj(t), where 1≤j≤s. Then{λ1, . . . , λs}is the set of all values of the spectrum ofCay(G, T).

Moreover, if mj is the multiplicity ofλj, then

mj = s

X

k=1

λk=λj

(χk(e))2.

Suppose Irr(G) denotes the set of all irreducible characters of Gand Cl(G) is the set of conjugacy classes ofG. We also assume that A(G) is the set of all normal subsets S of G\ {e}. Define S0 = G\(S∪ {e}). One can see that, S

is a minimal element of A(G) if and only if S0 is a second maximal element of A(G). Notice that the first maximal isG\ {e} and the Cayley graphCay(G, G\ {e}) is (|G| − 1)−regular. This implies that Cay(G, G\ {e}) is complete and

Spec(Cay(G, G\ {e})) ={−1(|G| −1 times), |G| −1}.

Proposition 2.2. Suppose G is a finite group with exactly n conjugacy classes
andS, S0 are normal subsets ofG\ {e} such thatS0 =G\S∪ {e}. Moreover, we
assume that Γ =Cay(G, S), Γ0 _{=} _{Cay}_{(}_{G, S}0_{)}_{. Set} _{Spec}_{(Γ) =} _{{}_{λ}

1, . . . , λn} and

Spec(Γ0_{) =}_{{}_{β}

1, . . . , βn}. Thenβi=−λi−1,1≤i≤n.

Proof. It is clear that S0 is a symmetric and generating subset of G. So, by Proposition 2.1,

βi = 1

χi(e)

X

s0_{∈S}0

χi(s0)

= 1

χi(e)

X

s0_{∈G}

χi(s0)−

X

s∈S∪{e}

χi(s)

= 1

χi(e)

X

s0_{∈G}

χi(s0)− 1

χi(e)

X

s∈S

χi(s)− 1

χi(e)

χi(e) = −λi−1,

proving the result.

Proposition 2.3. SupposeG={g1, g2, . . . , gn}andF is a subset of{1,2, . . . , n}

such that for all j ∈ F, the order of gj is a power of a prime pj. Define S =

S

j∈FgjG and Γ =Cay(G, S). We also assume that for eachχ∈Irr(G),χ(gj) is

an integer. Then Spec(Γ) ={λ1, . . . , λn}, where λi =P_{j∈F}|gGj|

1 + kjpj χi(e)

Proof. Supposeχ_{l}, 1 ≤ l ≤ |Irr(G)|, is an arbitrary irreducible character of G.
Then by [13, Corollary 22.27],χl(gj)≡χl(e) (mod pj). Thus,χl(gj)−χl(e) =pjkj,
for some integerkj. This implies that

χl(gj) χl(e) = 1 +

pjkj

χl(e) and so,

λl =

X

s∈S

χl(s)

χl(e)

=X

j∈F

X

s∈gG j

χl(s)

χl(e)

=X

j∈F

X

s∈gG j

1 + pjkj

χl(e)

=X

j∈F |gGj|

1 + pjkj

χl(e)

.

Corollary 2.4. SupposeG is a finite group,S is a normal subset ofGsuch that all elements of S are involutions andΓ =Cay(G, S). If F is a representative set forG−conjugacy classes ofS then the following statements hold:

1. if Gis abelian then allΓ−eigenvlues are odd or all of them are even, 2. if Gis a simple group thenλχ =Pg∈F|gG|(1 +

4kg χ(e)).

Proof. To prove (1), it is enough to apply Proposition 2.3 and the fact that the
irreducible characters ofG are linear. We now assume thatG is simple,g is an
involution in Gand χ ∈Irr(G). Then χ(g)≡χ(e) (mod 4) or G has a normal
subgroup of index2. SinceGis simple, there existskg such that χ(g)_{χ(1)} = 1 +

4kg χ(1). By Proposition 2.1,λχ =Pg∈S

χ(g) χ(e) =

P

x∈F

P

g∈xG(1 + 4kg χ(e))=

P

g∈F|gG|(1 + 4kg

χ(e)).

SupposeGis a group,A is the set of all character values ofG,Q(A)denotes the extension ofQ byA and Λ is the Galois group of this extension. It is well-known that there exists ε such thatQ(A)⊆Q(ε), where is a primitive n−th

root of unity. Thus, ifα ∈ Λ then there exists a unique positive integer r such
that (r, n) = 1 and α(ε) = εr_{. So, it is well define to use the notation} _{α} _{=}_{σ}

r. The group Λacts on the set of irreducible characters and conjugacy classes of G

byχα_{(}_{g}_{) =}_{α}_{(}_{χ}_{(}_{g}_{))}_{and}_{(}_{x}G_{)}σr _{= (}_{x}r_{)}G_{, respectively.}

Proposition 2.5. SupposeGis a finite group andS, T are twoΛ−conjugate con-jugacy classes of G. ThenΓ1=Cay(G, S)andΓ2=Cay(G, T)are co−spectral.

Proof. LetG be a finite group, χ be an irreducible character of G and S, T are two Λ−conjugate conjugacy classes of G. If{χ1, . . . , χr} is the orbit of χ under action of the Galois group, then {χ1(S), ..., χr(S)} = {χ1(T), ..., χr(T)}. Sup-pose Spec(Γ1) = {λ1, . . . , λr} and Spec(Γ2) = {µ1, . . . , µr}. If χi(S) = χi(T),

χi ∈ Irr(G), then λi = _{χ}_{i}1_{(e)}|T|χi(T) = _{χ}_{i}1_{(e)}|S|χi(S) = µi. In other case, if
{χn1, . . . , χns}is the orbit ofχiunder the action of Galois group then the values of

χn1(S), . . . , χnr(S)can be permuted to findχn1(T), . . . , χnr(T). This shows that

there exists positive integer ksuch that χi(e) =χk(e). Thus, λi = _{χ}_{i}1_{(e)}|T|χi(T)
= _{χ}1

k(e)|S|χk(S) = µk. Therefore, Γ1 = Cay(G, S) and Γ2 = Cay(G, T) are

The converse of Proposition 2.5 is not generally correct. To do this, we consider the following example:

Example 2.6. Consider the alternating groupA6which has exactly two conjugacy classesS andT of elements of order 3. It is easy to see that these classes are not Galois conjugate, but the Cayley graphsΓ1=Cay(G, S)andΓ2=Cay(G, T)are co−spectral. In fact,

Spec(Γ1) =

−8 −5 0 4 16 40

25 128 81 100 25 1

=Spec(Γ2).

Example 2.7. Consider the cyclic groupG =hai of ordern andS ={a, a−1}. ThenS is anM N Swith the following spectrum:

Spec(Cay(G, S)) ={wj+w−j |0≤j≤n−1},

where w = e2nπi. The energy of this Cayley graph approximately is 4n

π , see [9].
By a similar argument, one can proveT =S∪ {a2, a−2} is an SM N S forG, all
eigenvalues ofCay(G, T)are4Cos(3πj_{n} )Cos(πj_{n})),0≤j ≤n−1.

By method of subgroups of index 2 [13, p. 420], the group T4n has exactly

n+ 3conjugacy classes{e},{an_{},}_{{}_{a}r_{, a}−r_{}}_{(1}_{≤}_{r}_{≤}_{n}_{−}_{1),}_{{}_{a}2j_{b}_{|}_{0}_{≤}_{j} _{≤}_{n}_{−1}}
and {a2j+1_{b} _{|} _{0} _{≤}_{j} _{≤}_{n}_{−}_{1}. It is easy to see that}_{T}

4n has exactly four linear characters andn−1 non-linear irreducible characters recorded in Table 1. Example 2.8. An M N S of T4n can be computed as S = (ab)T4n∪bT4n and

S=aT4n_{∪}_{b}T4n_{, when} _{n}_{is odd and even, respectively. The simple eigenvalues of}

Cayley graphCay(T4n, S)are{n+2,−n−2, n−2,−n+2}and{2n,−2n}, for even
and odd n, respectively. Moreover, if n is even then this graph has eigenvalues
2Cos(πj_{n})with multiplicity four, where1≤j ≤n−1. Ifnis odd, 0 is an eigenvalue
ofCay(T4n, S)with multiplicity4n−2. To see this, it is easy to see thatS is an

M N SGofT4n. By Proposition 2.1, ifnis even thenλχi= 1

χi(e)(2χi(a) +nχi(b)).

Ifnis odd, thenλχi = n

χi(e)(χi(b)+χi(ab)). So, by Table 1, one can easily compute

the eigenvalues ofCay(T4n, S). If nis odd thenE(Cay(T4n, S)) = 4nand ifnis even then the energy of this Cayley graph is computed as follows:

E(Cay(T4n, S)) = 4n+ 8 n−1

X

j=1

|Cos(πj

n)|

≈ 4n+8n

π

Z π

0

|Cos(x)|dx−8 = 4n+16n

π −8.

Therefore we have

E(Cay(T4n, S))) ≈

4n if n is odd

By a similar argument, one can see thatT =S∪ {an_{}}_{is an}_{SM N S} _{for}_{T}
4n. Ifn
is odd then the simple eigenvalues ofCay(T4n, T)are 1±2n. Other eigenvalues
can be computed as−1 with multiplicity 2nand 1with multiplicity 2n−2. Ifn

is even then the simple eigenvalues ofCay(T4n, T)are3±nand −1±n. Other
eigenvalues can be computed as2Cos(πj_{n}) + (−1)j_{,}_{1}_{≤}_{j}_{≤}_{n}_{−1, with multiplicity}
4.

Example 2.9. The character table of V8n computed in [13, 421], when n is odd. Darafsheh and Poursalavati [6] generalized this group in the case that n

is even and computed its character table. This group has exactly 2n+ 3
conju-gacy classes, if n is odd, and 2n+ 6 conjugacy classes, for even n. If n is odd
then the conjugacy classes are: {1}, {b2}, {a2r+1, a−2r−1b2}(0 ≤ r ≤ n−1),
{a2s, a−2s}, {a2sb2, a−2sb2} (1 ≤ s ≤ n−1_{2} ), {ajbk | j is even & k = 1,3},
{ajbk | j is odd & k = 1,3}. The conjugacy classes, for even n, are: {1},
{b2_{},}_{{}_{a}n_{},}_{{}_{a}n_{b}2_{},}_{{}_{a}2r+1_{, a}−2r−1_{b}2_{}(0}_{≤}_{r}_{≤}_{n}_{−}_{1),}_{{}_{a}2s_{, a}−2s_{},}_{{}_{a}2s_{b}2_{, a}−2s_{b}2_{}}
(1 ≤ s ≤ n

2 −1), {a
2k_{b}(−1)k

| 0 ≤k ≤n−1}, {a2k_{b}(−1)k+1

| 0 ≤k ≤n−1},
{a2k+1b(−1)k|0≤k≤n−1},{a2k+1b(−1)k+1|0≤k≤n−1}. AnM N SofV8ncan
be computed atS =aV8n_{∪(}_{a}−1_{)}V8n_{∪}_{b}V8n_{and}_{S}_{=}_{a}V8n_{∪}_{b}V8n_{∪(}_{a}−1_{)}V8n_{∪(}_{b}−1_{)}V8n_{,}

when n is odd and even, respectively. The simple eigenvalues of Cayley graphs

Cay(V8n, S)are{±(2n+ 4),±(2n−4)}. Moreover these graphs have eigenvalues
0, with multiplicities4nand4Cos(πj_{n}),1≤j ≤n−1, with multiplicities four. By
our calculations, the energy ofV8n can be evaluated as follows:

E(Cay(V8n, S))≈

16 n= 1

8n+32n_{π} −16 otherwise.

A second minimal is T = S∪ {b2_{}. The simple eigenvalues of} _{Cay}_{(}_{V}

8n, T) are
5±2n and−3±2n. If nis even then other eigenvalues are−1 with multiplicity
4, 1 with multiplicity4(n−1) and 4Cos(πj_{n}) + 1, 1 ≤j ≤n−1, each of which
with multiplicity four. Ifnis odd then other eigenvalues are−1with multiplicity
4nand4Cos(πj_{n}) + 1,1≤j≤n−1, each of which with multiplicity four.

The group SD8n is presented bySD8n=ha, b|a4n=b2= 1 &bab=a2n−1i. In the following result the energy of the Cayley graph of this group with respect to its uniqueM N S is approximately computed.

Proposition 2.10. SupposeSis anM N SofSD8n. The energy ofE(Cay(SD8n), S)

can be evaluated as follows:

E(Cay(SD8n), S)≈

32n

π + 4n n= 1,3
8n+32n_{π} −16 otherwise.

Proof. All8nelements ofSD8nmay be given by{1, a, . . . , a4n−1, b, ba, . . . , ba4n−1}.
Following Hormozi and Rodtes [11], we define Ceven _{=} _{C}

1∪C2even∪C3even and

Codd_{=}_{C}

Ceven

3 = {2n+ 1,2n+ 3,2n+ 5, . . . ,3n−1}, C2odd = {1,3,5, . . . , n}, C3odd =
{2n+ 1,2n+ 3,2n+ 5, . . . ,3n}, C_{even}† =C1\ {0,2n} andC_{odd}† =C2even∪C3even.
Moreover, we assume thatCeven

? =Ceven\{0,2n}andC?odd=Codd\{0, n,2n,3n}. Then by [11, Proposition 2.2], the conjugacy classes ofSD8n,n≥2, can be com-puted as follows:

• Ifnis even, there are2n+ 3 conjugacy classes as follows:
– 2 classes of size one being[1] ={1}and[a2n_{] =}_{{}_{a}2n_{},}

– 2n−1 conjugacy classes of size two being[ar] ={ar, a(2n−1)r}, where

r∈C_{?}even,

– 2 classes of size 2n being [b] = {ba2t _{|} _{0} _{≤} _{t} _{≤} _{2}_{n}_{−}_{1}} _{and} _{[}_{ba}_{] =}
{ba2t+1 _{|}_{0}_{≤}_{t}_{≤}_{2}_{n}_{−}_{1}.}

• Ifnis odd, then there are2n+ 6conjugacy classes as follows:

– 4 classes of size one being [1] = {1}, [an_{] =}_{{}_{a}n_{},} _{[}_{a}2n_{] =}_{{}_{a}2n_{}} _{and}
[a3n_{] =}_{{}_{a}3n_{},}

– 2n−2classes of size two being [ar_{] =}_{{}_{a}r_{, a}(2n−1)r_{}, where}_{r}_{∈}_{C}odd
? ,
– 4 classes of sizenbeing[b] ={ba4t|0≤t≤n−1},[ba] ={ba4t+1|0≤

t≤n−1}, [ba2] ={ba4t+2 | 0≤t ≤n−1} and[ba3] = {ba4t+3 | 0≤

t≤n−1}.

AnM N S of SD8n can be computed as S = (a)SD8n∪(a−1)SD8n∪bSD8n. This
graph has simple eigenvalues ±(4 + 2n) and ±(4−2n). If n is even then other
eigenvalues are4Cos(hπ_{2n}), where h∈ C_{even}† , each of which with multiplicity four
and 0 with multiplicity 4n. If n is odd, other eigenvalues are 4Cos(hπ_{2n}), where

h∈C_{even}† , each of which with multiplicity four,±n and0with multiplicities two
and4n−8 respectively. By [19, Theorem 1], ifnis even thenλχi =

2

χi(e)(χi(a) +

χi(a−1) +nχi(b)). If nis odd, then λχi = 1

χi(e)(2χi(a) + 2χi(a

−1_{) +}_{nχ}

i(b)). By these calculations and a similar argument as Example 2.8, the energy ofSD8n can be computed as follows:

E(Cay(SD8n), S)≈

32n

π + 4n n= 1,3
8n+32n_{π} −16 otherwise.

which proves the result.

Example 2.11. In this example, the eigenvalues of Cay(G, S) are computed, where G is a Mathieu group and S is an M N S or SM N S for G. Since G is a simple group, hAi = G, if A is a conjugacy class of G. So, if a real conjugacy classS ofGhas the minimum size between all conjugacy classes ofGthenSis an

M N SforG. It is easy to see that for each Mathieu group, the conjugacy class2A

real conjugacy classes of these groups and their union. The conjugacy class3Ain

M11, M22andM23is again real and has the second size between all real conjugacy classes of these groups and their union. These conjugacy classes areSM N S ofG

and all eigenvalues with respect to these classes are recorded in Tables 2−6. For the sake of completeness, we present here some details on the Suzuki group. Following Suzuki [16], a group G is called a ZT−group if G acts on a set Ω in such a way that, (1) G is a doubly transitive group on 1 +N symbols, (2) the identity is the only element which leaves three distinct symbols invariant, (3) G

contains no normal subgroup of order 1 +N, and (4) N is even. Suzuki [16]
proved that for each prime powerq = 22s+1_{, there is a unique} _{ZT}_{−group} _{Sz}_{(}_{q}_{)}
of order q2_{(}_{q}_{−}_{1)(}_{q}2_{+ 1)} _{which is called later the Suzuki group. This group is}
simple, whenq > 2. Suppose that ais a symbol on which G acts andH =Ga.
By [16], it follows from the conditions (1) and (2) thatH is a Frobenius group
onΩ\ {a}. Apply a well-known result of Frobenius to deduce thatH contains a
regular normal subgroupQof orderN such that every non-identity element ofQ

leaves only the symbola invariant. Suppose b ∈Ω\ {a} and K =Hb. Suppose

x ∈ NG(K) is an involution. Then it is well-known that the Suzuki group are
containing two elements y andz such thaty is an involution and xyx=z−1xz,
and three cyclic subgroupsA0,A1andA2 of ordersq−1,q+r+ 1andq−r+ 1,
respectively. By [16], the conjugacy classes ofSz(q)can be computed as follows:
{e},ySz(q)_{,}_{z}Sz(q)_{,}_{(}_{z}−1_{)}Sz(q)_{,}_{b}Sz(q)

0 ,b Sz(q)

1 andb

Sz(q)

2 of lengths 1,(q−1)(q2+ 1), 1

2q(q−1)(q

2_{+ 1),} 1

2q(q−1)(q

2_{+ 1),}_{q}2_{(}_{q}_{−}_{1)(}_{q}_{+}_{r}_{+ 1),}_{q}2_{(}_{q}_{+}_{r}_{+ 1)(}_{q}_{−}_{r}_{+ 1)}
andq2_{(}_{q}_{−}_{1)(}_{q}_{−}_{r}_{+ 1), respectively. Here,}_{b}

0, b1andb2are non-identity elements
ofAi, i= 0,1,2, respectively. Note that there are q−r_{4} , q_{2}−1 and q+r_{4} conjugacy
classes of typesbSz(q)_{0} ,bSz(q)_{1} andbSz(q)_{2} , respectively.

One can also find the character table of this group in [16]. Hence, by applying above information on Suzuki groups and Proposition 2.1 we have the following proposition:

Proposition 2.12. Consider the Suzuki group Sz(q) with q = 22s+1_{,} _{r} _{= 2}s+1

and s≥1. The conjugacy class S =ySz(q) _{and the normal subset} _{T} _{=}_{z}Sz(q)_{∪}
(z−1)Sz(q) are the M N S and SM N S of Sz(q), respectively. Moreover, |S| =
(q−1)(q2+ 1),|T|=q(q−1)(q2+ 1)and the simple eigenvalues ofCay(Sz(q), S)

andCay(Sz(q), T)are |S|and|T|, respectively. The Cayley graphCay(Sz(q), S)

has eigenvalues0,−(q2_{+1)}_{,}_{(}_{q}_{−1)}_{,}(1+q2_{)(r−1)}

q−r+1 and

−(1+q2_{)(r+1)}

q+r+1 with multiplicities

q4, (q−1)_{2}2(r2), q−2_{2} (q2+ 1)2, q+r_{4} ((q−r+ 1)(q−1))2 and q−r_{4} ((q+r+ 1)(q−1))2,
respectively. The energy ofCay(Sz(q), S)is as follows:

E(Cay(Sz(q), S)) = − √

2q

2 +

p

2q3

2 −

p

2q7_{+}

p

2q5

2 +

3p2q9

2 −

3p2q11 2 +

p

2q13 2 −q

5_{+}

p q2 2 −q

3_{+}_{q}4_{+}

The Cayley graphCay(Sz(q), S)has eigenvalues0,q(q−1),−q(q_{q−r+1}2+1)and−q(q_{q+r+1}2+1)

with multiplicities 1 4(4q

4_{+}_{r}2_{(}_{q}_{−1)}2_{)}_{,} 1

2(q−2)(q

2_{+ 1)}2_{,} 1

4(q−r+ 1)

2_{(}_{q}_{−1)}2_{(}_{q}_{+}_{r}_{)}

and 1

4(q+r+ 1)

2_{(}_{q}_{−}_{1)}2_{(}_{q}_{−}_{r}_{)}_{, respectively.}

We end our paper by computing eigenvalues of a group of type Ree of charac-teristicq. We refer to [15, 16, 18] for our notations and known results concerning this important class of simple groups. A finite groupGhas Ree type ifGsatisfies the following conditions:

• The Sylow2−subgroups ofGare elementary abelian of order 8.

• The groupGhas no normal subgroup of index 2.

• There is an involution J in G such that the centralizer CG(J) ∼= hJi ×

LF(2, q), whereL=LF(2, q)denotes the linear fractional group onGF(q).

• If hRi denotes a cyclic subgroup of order q+e_{2} in L, then for any subgroup
16=hR0i, we haveNG(hR0i)≤CG(J).

• Let J0 _{be an involution of} _{L} _{and} _{S} _{an element of} _{L} _{of order} q−e
4 which
centralizes J0_{. Then an element of} _{G} _{of order} _{3} _{which normalizes} _{h}_{J, J}0_{i}
does not centralizeS. We callqthe characteristic ofG.

Note that the (I) implies thatq≡4 +e(mod8)where e=±1. In the end of this paper, we consider the simple Ree group G2(q)of characteristic q and order

q3(q−1)(q3+ 1), where q = 32k+1 and k ≥ 1. This group has exactly q+ 8 conjugacy classes [18]. Supposem= 3k. Then we have:

Proposition 2.13. The conjugacy classS=JG2(q)_{is the unique}_{M N S} _{of}_{G}

2(q)

with size q(q_{q}22−q+1)_{+1} . The energy ofCay(G2(q), S)can be computed by the following

formula:

E(Cay(G2(q), S)) =

1

4(1 + 34k+2_{)}(19·3
22k

+ 4e38k+ 34k+3+ 32k+1+ (2e−11)321k + 2e32k+1+ 325k+ 280·318k+ 313k−4·310k)

+ 4

(32k+1_{+ 3}k+1_{+ 1)(1 + 3}4k+2_{)}(3
22k

+ 2·319k+ 20·316k+ 38k

− 11·310k+ 14·313k).

conjugacy classes ofG2(q)are as follows:

q3(q2−q+ 1),q

3_{(}_{q}_{−}_{1)(}_{q}3_{+ 1)}

q2_{+ 1} ,

q3_{(}_{q}_{−}_{1)(}_{q}3_{+ 1)}

q2_{−}_{3}q+2_{2} ,

q3(q−1)(q3+ 1)

q2_{+ 3}q+2_{2} ,

(q−1)(q3+ 1)

q2 ,

q(q−1)(q3+ 1)

2 ,

(q−1)(q3_{+ 1)}

2q ,

q(q−1)(q3_{+ 1)}

3 ,

q(q2_{−}_{q}_{+ 1)}

q2_{+ 1} .

By above information on conjugacy lengths ofG2(q)and some tedious calculations,
one can see thatS=JG2(q)_{is the unique}_{M N S} _{of}_{G}

2(q). The simple eigenvalues ofCay(G2(q), S)is|S|. This graph has eigenvalues

0,− q
q2_{+1},

3q
q2_{+1},

q
q2_{+1},

q(q2−q+1)
m(q2_{+1)(q+3m+1)},

q(q2−q+1)
m(q2_{+1)(q−3m+1)}

with multiplicities,314k+6_{−3}12k+5_{−}_{5·3}10k+4_{−}_{7·3}6k+2_{+}_{2·3}8k+3_{−}_{3}4k+1_{−3}2k+1_{,}
1

8·3

14k+8 _{−}11
8 ·3

12k+6_{+} e
4·3

12k+6_{+} 5
2·3

10k+5 _{−}11
4 ·3

8k+4_{+} 7
4·3

6k+3_{−} 1
2·3

4k+2

− 1 8·3

2k+1_{+} 1
8 +

e 2·3

6k+3_{+} e
4,

1 8·3

14k+6_{−} 7
8·3

12k+5 _{+}5
2·3

10k+4_{−} 17
4 ·3

8k+3 _{+}
19

4 ·3

6k+2_{−} 7
2·3

4k+1 _{+} 13
8 ·3

2k _{−} 1
8,

1 43

14k+7_{−} 1
2·3

12k+6 _{+} e
4·3

12k+6_{+} 1
2·3

8k+4

− 36k+3 + e_{2} ·36k+3 + 1_{4} ·32k+1 − 1
2 +

e 4, 2·3

10k+4 _{+} _{4}_{·}_{3}9k+4 _{+} _{2}_{·}_{3}8k+4 _{−}
4·37k+3 _{−} _{16}_{·}_{3}6k+2 _{−}_{4}_{·}_{3}5k+2 _{+} _{2}_{·}_{3}4k+2 _{+} _{4}_{·}_{3}3k+1 _{+} _{2}_{·}_{3}2k_{,} _{2}_{·}_{3}10k+4 _{−}
4·39k+4 _{+}_{2}_{·}_{3}8k+4_{+}_{4}_{·}_{3}7k+3 _{−}_{16}_{·}_{3}6k+2 _{+}_{4}_{·}_{3}5k+2 _{+}_{2}_{·}_{3}4k+2 _{−}_{4}_{·}_{3}3k+1 _{−}
2·32k_{, respectively. Therefore, the energy of}_{Cay}_{(}_{G}

2(q), S)is as follows: E(Cay(G2(q), S)) =

1

4(1 + 34k+2_{)}(19·3
22k

+ 4e38k+ 34k+3+ 32k+1+ (2e−11)321k + 2e32k+1+ 325k+ 280·318k+ 313k−4·310k)

+ 4

(32k+1_{+ 3}k+1_{+ 1)(1 + 3}4k+2_{)}(3
22k

+ 2·319k+ 20·316k+ 38k

− 11·310k+ 14·313k).

This completes the proof.

By the notation of [18] we have the following proposition.

Proposition 2.14. The conjugacy class T = XG2(q) _{is the unique} _{SM N S} _{of}

G2(q) with size (q−1)(q

3_{+1)}

q2 . The simple eigenvalues of Cay(G2(q), T) is|T| and

the graph has the eigenvalues

0,−(q−1)_{q}2_{(q}2_{−q+1)}2(q3+1),

(q−1)(q3_{+1)}

q2_{(q}2_{−q+1)},

−(q+m)(q3_{+1)}

mq2_{(q+3m+1)},

(q+m)(q3_{+1)}

mq2_{(q−3m+1)},

−(q3_{+1)}

q2_{(q+1)},

q−1 q2 ,

(2q−1)(q3_{+1)}

q2_{(q}2_{−q+1)} ,

−(q+1+3m)(q3_{+1)}

(q+1)(q+1+3m)q2,

−(q+1−3m)(q3_{+1)}

with multiplicities,q6_{,}_{3}8k+4_{−}_{2}_{·}_{3}6k+3_{+ 3}4k+3_{−2}_{·}_{3}2k+1_{+ 1}_{,}_{3}12k+6_{−2}_{·}_{3}10k+5_{+}
38k+5_{−2·3}6k+3_{,} 1

2·3

10k+4_{+}1
2·3

8k+4_{+3}9k+4_{−4}_{·3}6k+4_{−3}7k+3_{+}1
2·3

4k+2_{−3}5k+2_{+}1
2·
32k_{+3}3k+1_{+3}3k+1_{,} 1

2·3

10k+4_{+3}9k+4_{+}1
2·3

8k+4_{+3}7k+3_{−4·3}6k+2_{+3}5k+2_{+}1
2·3

4k+2_{−}
33k+1_{+}1

2·3

2k_{,}_{2·3}10k+4_{−4·3}6k+2_{+2·3}2k_{,} 1
2·3

14k+7_{−}1
2·(1+

e 2)·3

12k+6_{+3}8k+4_{+(}_{e}_{−}
2)·36k+3_{+}1

2·3

2k+1_{−1+}e
2,

1 2(3

14k+6_{−7·}_{3}12k+5_{+20·3}10k+4_{−34·3}8k+3_{+38·3}6k+2_{−}
28·34k+1_{+13·3}2k_{−1)}_{,} 1

2·3

14k+6_{−}7
2·3

12k+5_{+10·3}10k+4_{−17·3}8k+3_{+19·3}6k+2_{−14·}
34k+1 13

2·3
2k_{−}1

2, 1 2·3

14k+6_{+}1
2·3

13k+7_{+}11
2 ·3

12k+5_{+}7
2·3

11k+5_{+}5
2·3

10k+4_{−}5
2·3

9k+4_{−}
11·38k+3−7·337k+3−13

2 ·3

6k+2_{+}1
2·3

5k+2_{+}11
2 ·3

4k+1_{+}7
2·3

3k+1_{+}7
2·3

2k_{+}1
2·3

k_{,}

1 2·3

14k+6_{−}1
2·3

13k+6_{−}11
2 ·3

12k+5_{+}1
2·3

11k+6_{−}7
2·3

10k+4_{+}1
2·3

9k+5_{+ 3}8k+3_{−}
37k+4+11_{2} ·36k+2−1

2·3

5k+3_{−}1
2·3

4k+1_{+}1
2·3

3k+2_{−}5
2·3

2k_{+}1
2·3

k_{, respectively.}

Table 1: The Character Table ofT4n.

gi e an ar(1≤r≤n−1) b ab

|CG(gi)| 4n 4n 2n 4 4

Non-Linear Characters

ψj (1≤j ≤n−1) 2 2(−1)j wrj+w−rj 0 0

Linear Characters n is odd

χ1 1 1 1 1 1

χ2 1 −1 (−1)r i −i

χ3 1 1 1 −1 −1

χ4 1 −1 (−1)r −i i

Linear Characters n is even

λ1 1 1 1 1 1

λ2 1 1 1 −1 −1

λ3 1 1 (−1)r 1 −1

λ4 1 1 (−1)r −1 1

Table 2: The Eigenvalues ofCay(M11,2A)and Cay(M11,3A)

2A−Eigenvalues M ultiplicities 3A−Eigenvalues M ultiplicities

−33 23·52 −55 29

−11 34·52 −10 24·112

−3 52_{·}_{11}2 _{0} _{3}4_{·}_{5}2

0 29 _{8} _{5}2_{·}_{11}2

15 24_{·}_{11}2 _{44} _{2}2_{·}_{3}_{·}_{5}2

33 22_{·}_{5}2 _{80} _{11}2

45 112 _{440} _{1}

Table 3: The Eigenvalues ofCay(M12,2A)andCay(M12,2B).

2A−Eigenvalues M ultiplicities 2B−Eigenvalues M ultiplicities

−36 7·113 _{−33} _{3}2_{·}_{5}2_{·}_{73}

−9 28_{·}_{11}2 _{−9} _{2}_{·}_{5}2_{·}_{11}2

−4 34_{·}_{11}2 _{0} _{3}4_{·}_{5}2

0 26_{·}_{3}2_{·}_{5}2 _{15} _{3}2_{·}_{11}2_{·}_{13}

11 28·34 55 22·36

36 22·32·112 63 52·112

44 34·61 135 2·112

99 29 495 1

396 1 − −

Table 4: The Eigenvalues ofCay(M22,2A)andCay(M22,3A).

2A−Eigenvalues M ultiplicities 3A−Eigenvalues M ultiplicities

−77 2·34_{·}_{5}2 _{−160} _{3}2_{·}_{7}2_{·}_{11}2

−33 27_{·}_{5}2_{·}_{7}2 _{−64} _{5}2_{·}_{7}2_{·}_{11}2

3 52_{·}_{7}2_{·}_{11}2 _{0} _{3}6_{·}_{19}

11 22_{·}_{3}2_{·}_{5}2_{·}_{7}2 _{44} _{2}7_{·}_{5}2_{·}_{7}2

35 34_{·}_{11}2 _{80} _{2}2_{·}_{7}2_{·}_{11}2

75 22_{·}_{7}2_{·}_{11}2 _{224} _{5}2_{·}_{11}2

147 52_{·}_{11}2 _{1760} _{3}2_{·}_{7}2

275 32_{·}_{7}2 _{12320} _{1}

Table 5: The Eigenvalues ofCay(M23,2A)andCay(M23,3A).

2A−Eigenvalues M ultiplicities 3A−Eigenvalues M ultiplicities

−253 2·34_{·}_{5}2 _{−736} _{2}_{·}_{3}2_{·}_{7}2_{·}_{11}2
−69 24_{·}_{5}3_{·}_{11}2_{·}_{13} _{−253} _{2}15_{·}_{7}2

0 21_{5}_{·}_{7}2 _{−28} _{2}6_{·}_{11}2_{·}_{23}2

15 26_{·}_{11}2_{·}_{23}2 _{0} _{3}4_{·}_{5}2_{·}_{1499}

99 34_{·}_{5}2_{·}_{23}2 _{224} _{11}2_{·}_{23}2

115 33_{·}_{7}2_{·}_{11}2 _{368} _{2}3_{·}_{5}2_{·}_{7}2_{·}_{11}2

195 112_{·}_{23}2 _{1232} _{2}2_{·}_{5}2_{·}_{23}2

363 22·52·232 1472 32·72·112

1035 22·112 10304 22·112

3795 1 56672 1

Table 6: The Eigenvalues ofCay(M24,2A)andCay(M24,2B).

2A−Eigenvalues M ultiplicities 2B−Eigenvalues M ultiplicities

−759 2·34_{·}_{5}2 _{−1386} _{2}_{·}_{23}2_{·}_{61}

−231 2·34·52·232 −1242 2·32·72·112 −207 24·53·112·13 −378 52·112·232 −135 72·112·232 −322 23·34·52·112 −115 26·34·72·112 −266 34·112·232

−55 24·34·72·232 −154 25·52·232
−23 36_{·}_{5}2_{·}_{7}2_{·}_{11}2 _{−138} _{3}6_{·}_{5}2_{·}_{7}2_{·}_{11}2

45 26_{·}_{11}2_{·}_{23}2 _{0} _{2}12_{·}_{5}2_{·}_{11}2

105 23_{·}_{3}2_{·}_{11}_{·}_{23}_{·}_{29}_{·}_{2783} _{54} _{3}2_{·}_{7}2_{·}_{11}2_{·}_{23}2
165 28_{·}_{3}4_{·}_{23}2 _{138} _{2}6_{·}_{3}4_{·}_{7}2_{·}_{11}2

207 212_{·}_{5}2_{·}_{11}2 _{154} _{2}8_{·}_{3}4_{·}_{23}2

345 2·32_{·}_{7}2_{·}_{11}2 _{198} _{2}2_{·}_{5}2_{·}_{23}2

297 34_{·}_{5}2_{·}_{23}2 _{378} _{2}6_{·}_{11}2_{·}_{23}2

585 112_{·}_{23}2 _{414} _{2}3_{·}_{5}2_{·}_{7}2_{·}_{11}2

297 34_{·}_{5}2_{·}_{23}2 _{1078} _{2}4_{·}_{5}2_{·}_{23}2

441 52_{·}_{11}2_{·}_{23}2 _{1518} _{2}4_{·}_{3}4_{·}_{7}2

585 112·232 3542 34·52

1265 24·34·72 − −

3465 232 − −

Acknowledgement. The author is indebted to professor Marty Isaacs for some discussion on this paper and for giving the counterexample given Example 2.6. This paper is a part of my PhD thesis supervised under the direction of Professor Ali Reza Ashrafi at the University of Kashan.

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Maryam Jalali-Rad

Department of Pure Mathematics, Faculty of Mathematical Sciences, University of Kashan,