1,3

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.

P. J. Davis, Circulant Matrices, J. Wiley, New York, 1979.

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).

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.

n=4: the eigenvalues of M(4) are therefore: 0*I + 1*(-1) + 1*(-I) + 2*1 = 1-I, 2, 1+I, 4.

n=4: a(4)= Det(M(4)) = 16 = (1-I)*2*(1+I)*4.

Cf. A123745 (other Fibonacci circulants without F_0 = 0).

Sequence in context: A009764 A000182 A102599 this_sequence A136796 A055546 A009549

Adjacent sequences: A123741 A123742 A123743 this_sequence A123745 A123746 A123747

nonn,easy

Wolfdieter Lang (wolfdieter.lang(AT)physik.uni-karlsruhe.de) Nov 10 2006