OFFSET
3,2
REFERENCES
L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 159.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 3..448
FORMULA
a(n) = (n!/6) * Sum_{i,j,k > 0 and i+j+k=n} sigma(i)*sigma(j)*sigma(k)/(i*j*k). - Seiichi Manyama, Nov 09 2020.
E.g.f.: -(1/6) * log( Product_{k>=1} (1 - x^k) )^3. - Ilya Gutkovskiy, Apr 24 2021
MATHEMATICA
nmax = 20; Table[n!/6 * Sum[Sum[Sum[If[i + j + k == n, DivisorSigma[1, i] * DivisorSigma[1, j] * DivisorSigma[1, k] / (i*j*k), 0], {k, 1, n}], {j, 1, n}], {i, 1, n}], {n, 3, nmax}] (* Vaclav Kotesovec, Nov 09 2020 *)
PROG
(PARI) {a(n) = my(t='t); n!*polcoef(polcoef(prod(k=1, n, (1-x^k+x*O(x^n))^(-t)), n), 3)} \\ Seiichi Manyama, Nov 07 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jan 27 2001
EXTENSIONS
More terms from Vladeta Jovovic, Dec 28 2001
STATUS
approved