OFFSET
1,3
COMMENTS
A circulant C_n is the determinant of a circulant n X n matrix M, i.e. one with entries M_{i,j}=a_{i-j} where the indices are taken mod n. Hence C_n=C_n([a_n,a_{n-1},...,a_1]), with the first row of M given.
The eigenvalues of a circulant n X n matrix M(n) are lambda^{(n)}_k=sum(a_j*(rho_n)^(j*k),j=1..n), with the n-th roots of unity (rho_n)^k, k=1..n, where rho_n:=exp(2*Pi/n). See the P. J. Davis reference which uses a different convention.
REFERENCES
P. J. Davis, Circulant Matrices, J. Wiley, New York, 1979.
FORMULA
a(n) = product(lambda^{(n)}_k,k=1..n), with lambda^{(n)}_k=sum(F_{j-1}*(rho_n)^(j*k),j=1..n).
a(n) = C_n([F_{n-1},F_{n-2},...,F_0]) with the Fibonacci numbers F_n:=A000045(n) and the circulant C_n (see comment above).
EXAMPLE
n=4: the circular 4 X 4 matrix is M(4) = matrix([[2,1,1,0],[0,2,1,1],[1,0,2,1],[1,1,0,2]]).
n=4: 4th roots of unity: rho_4 = I, (rho_4)^2 = -1, (rho_4)^3 = -I, (rho_4)^4 =1, with I^2=-1. A123744 n=4: the eigenvalues of M(4) are therefore: 0*I^k + 1*(-1)^k + 1*(-I)^k + 2*1^k, k=1,...,4, namely 1-I, 2, 1+I, 4.
n=4: a(4)= Det(M(4)) = 16 = (1-I)*2*(1+I)*4.
PROG
(PARI) mm(n) = matdet(matrix(n, n, i, j, fibonacci(n-1-lift(Mod(j-i, n))))); \\ Michel Marcus, Aug 11 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Nov 10 2006, Jan 27 2009
EXTENSIONS
More terms from Michel Marcus, Aug 11 2019
STATUS
approved