Theorem 2. There are .n − 2/=8 non-zero numbers m so that the eigenvalues of
A.Sn/ are p; −p and .n − 2/=8 octets of eigenvalues each containing two copies of
each of m and
−1
m ; m D 1; : : : ; .n − 2/=8; and each of the latter eigenvalues
has a magnitude less than the spectral radius p.
Proof. Since p is odd, p D 2q C 1, and so .n − 2/=8 D .p2 C 1 − 2/=8 D .q2 C
q/=2, which is an integer for every integer value of q. This determines the number
of octets characterized below for each prescribed value of n. Since A.Sn/ is nonnegative,
irreducible and symmetric, all eigenvalues are real, and Perron–Frobenius
theory implies that the two eigenvalues p and −p are simple, and all other eigenvalues
have magnitude less than p. As each of these remaining eigenvalues E.z/ in
(3) is a real valued function of z with real coefficients, and z is an nth root of unity,
E.z/ D E.z/ D E.z/ D E.1=z/. This shows that each remaining eigenvalue occurs
twice. Also, by (3), E.−z/ D −E.z/, which shows that if is an eigenvalue, then
− is also an eigenvalue. The proof of Theorem 1 shows that if =D p, then
−1
is also an eigenvalue. Combining these observations gives the octet result.