A347929 - OEIS

A347929

a(n) = 2^(-1 + (n + n mod 2)/2)*abs(permanent(M_n)) where M_n is the n X n matrix M_n(j, k) = cos(Pi*j*k/n) if n >= 1 and a(0) = 1.

1, 1, 1, 2, 0, 6, 12, 12, 96, 108, 240, 380, 0, 28428, 8176, 16200, 387072, 2817324, 6065280, 4604796, 56832000, 14574168, 2092107072, 13994428360, 8725045248, 162749055000, 1304167707648, 3291435901044, 17899142381568, 107056050266172

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OFFSET

0,4

LINKS

Table of n, a(n) for n=0..29.

Zhi-Wei Sun, Fedor Petrov, A surprising identity, discussion in MathOverflow, Jan 17 2019.

Zhi-Wei Sun, On some determinants involving the tangent function, arXiv:1901.04837 [math.NT], 2021.

EXAMPLE

a(6) = 16*cos^4(Pi/8) + 8*cos^2(Pi/8) - 64*cos^2(Pi*3/8)*cos^2(Pi/8) + 8*cos^2(Pi*3/8) + 16*cos^4(Pi*3/8).

PROG

(SageMath)

def A347929(n):

if n == 0: return 1

RF = RealField(100) # adjust precision if needed

M = matrix(RF, n, n, lambda j, k: cos(j * k * pi / n))

c = 2^(-1 + (n + n % 2) // 2)

return abs(round(c*M.permanent()))

print([A347929(n) for n in range(12)])

(PARI)

p(n) = matpermanent(matrix(n, n, j, k, cos((Pi*j*k)/n)));

A347929(n) = abs(round(2^(-1 + (n + n %2)/2)*p(n)));

{for(n = 0, 12, print(A347929(n)))}

CROSSREFS

Cf. A347281.

Sequence in context: A350462 A357367 A110667 * A129877 A371913 A307581

Adjacent sequences: A347926 A347927 A347928 * A347930 A347931 A347932

KEYWORD

nonn

AUTHOR

Peter Luschny, Sep 19 2021

STATUS

approved