OFFSET
1,2
LINKS
Michel Marcus, Table of n, a(n) for n = 1..500
FORMULA
a(n) = 3 iff n is prime. - Bernard Schott, Oct 17 2021
EXAMPLE
a(6) = 50; a(6) = sigma_[6-1](1) + sigma_[6-2](2) + sigma_[6-3](3) + sigma_[6-6](6) = (1^5) + (1^4 + 2^4) + (1^3 + 3^3) + (6^0 + 6^0 + 6^0 + 6^0) = 50.
MATHEMATICA
a[n_] := DivisorSum[n, DivisorSigma[n - #, #] &]; Array[a, 30] (* Amiram Eldar, Oct 17 2021 *)
PROG
(PARI) a(n) = sumdiv(n, d, sigma(d, n-d)); \\ Michel Marcus, Oct 18 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Oct 16 2021
STATUS
approved