OFFSET
1,2
LINKS
A. Alaca, S. Alaca, K. S. Williams, Some Infinite Products of Ramanujan Type, Canad. Math. Bull. Vol. 52 (4), 2009, pp. 481-492, [p. 491, equation (5.1)]
FORMULA
a(n) is multiplicative and a(7^e) = 1, a(p^e) = ((p^2)^(e+1) - 1) / (p^2 - 1) if p == 1, 2, 4 (mod 7), a(p^e) = (-(-p^2)^(e+1) + 1) / (p^2 + 1) if p == 3, 5, 6 (mod 7).
G.f.: Sum_{k>0} x^k * Sum_{d|k} d^2 * Kronecker(-7, d). - Michael Somos, Feb 24 2011
-7*a(n) = A138809(n) unless n = 0.
EXAMPLE
G.f. = q + 5*q^2 - 8*q^3 + 21*q^4 - 24*q^5 - 40*q^6 + q^7 + 85*q^8 + 73*q^9 + ...
PROG
(PARI) {a(n) = if( n<1, 0, sumdiv( n, d, d^2 * kronecker( -7, d)))};
(PARI) {a(n) = my(A, p, e); if( n<1, 0, A = factor(n); prod( k=1, matsize(A)[1], [p, e] = A[k, ]; if( p==7, 1, kronecker( p, 7)==1, ((p^2)^(e+1) - 1) / (p^2 - 1), (-(-p^2)^(e+1) + 1) / (p^2 + 1)))) };
CROSSREFS
KEYWORD
sign,mult
AUTHOR
Michael Somos, Mar 31 2008
STATUS
approved