A279888 -id:A279888 - cp's OEIS frontend

A279917 a(n) = Sum_{k=1..n-1} sigma_5(k)*sigma_7(n-k).

0, 1, 162, 6689, 121250, 1296406, 9613604, 54550049, 252178758, 992204376, 3424910566, 10615778966, 30047257318, 78751366604, 193075366412, 446840757793, 982597838280, 2066025009763, 4171924730922, 8127978871064, 15324653827568, 28059983210370

Offset: 1

Seiichi Manyama, Dec 23 2016

nonn

Seiichi Manyama, Table of n, a(n) for n = 1..1000

Cf. A001160, A013955, A013961, A279888.

a[n_] := (DivisorSigma[13, n] + 20 * DivisorSigma[7, n] - 21 * DivisorSigma[5, n]) / 10080; Array[a, 25] (* Amiram Eldar, Jan 07 2025 *)

a(n) = {my(f = factor(n)); (sigma(f, 13) + 20 * sigma(f, 7) - 21 * sigma(f, 5)) / 10080;} \\ Amiram Eldar, Jan 07 2025

a(n) = (sigma_13(n)+20*sigma_7(n)-21*sigma_5(n))/10080.

A279926 a(n) = Sum_{k=1..n-1} sigma_3(k)*sigma_9(n-k).

Original entry on oeis.org

0, 1, 522, 24329, 454250, 4905766, 36532244, 207705929, 961214238, 3784166376, 13066960126, 40511160326, 114681233758, 300599979884, 737035375772, 1705830324553, 3751239987240, 7887626314003, 15927815870322, 31031953887704, 58508991327728, 107133058597170

Offset: 1

Seiichi Manyama, Dec 23 2016

nonn

Seiichi Manyama, Table of n, a(n) for n = 1..1000

Cf. A001158, A013957, A013961, A279888, A279917.

Table[Sum[If[k == 0, 0, DivisorSigma[3, k]] DivisorSigma[9, n - k], {k, 0, n - 1}], {n, 22}] (* Michael De Vlieger, Dec 23 2016 *)

a(n) = sum(k=1, n-1, sigma(k, 3)*sigma(n-k, 9)) \\ Felix Fröhlich, Dec 23 2016

a(n) = (sigma_13(n) - 11*sigma_9(n) + 10*sigma_3(n))/2640.

cp's OEIS Frontend

A279917 a(n) = Sum_{k=1..n-1} sigma_5(k)*sigma_7(n-k).

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