OFFSET
0,2
LINKS
Amiram Eldar, Table of n, a(n) for n = 0..10000
K. S. Williams, The parents of Jacobi's four squares theorem are unique, Amer. Math. Monthly, 120 (2013), 329-345.
MATHEMATICA
s[n_, k_] := If[Divisible[n, k], DivisorSigma[1, n/k], 0]; a[0] = 1; a[n_] := 2*s[n, 1] - 3*s[n, 2] + 4*s[n, 4] + 9*s[n, 6] - 36*s[ n, 12]; Array[a, 100, 0] (* Amiram Eldar, Aug 17 2019 *)
PROG
(PARI) s(n, k) = if (!(n%k), sigma(n/k), 0);
a(n) = if (n==0, 1, 2*s(n, 1)-3*s(n, 2)+4*s(n, 4)+9*s(n, 6)-36*s(n, 12)); \\ Michel Marcus, Sep 27 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Apr 09 2013
STATUS
approved