OFFSET
1,3
FORMULA
a(n) = (n-1)! * Sum_{k=1..n-1} d(k)*d(n-k)/k.
MATHEMATICA
a[n_] := (n - 1)! * Sum[DivisorSigma[0, k] * DivisorSigma[0, n - k]/k, {k, 1, n - 1} ]; Array[a, 22] (* Amiram Eldar, Nov 10 2020 *)
PROG
(PARI) {a(n)= n!*sum(k=1, n-1, numdiv(k)*numdiv(n-k)/(k*(n-k)))/2}
(PARI) {a(n)= (n-1)!*sum(k=1, n-1, numdiv(k)*numdiv(n-k)/k)}
(PARI) {a(n) = my(u='u); n!*polcoef(polcoef(prod(k=1, n, (1-x^k+x*O(x^n))^(-u/k)), n), 2)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Nov 10 2020
STATUS
approved