OFFSET
1,2
COMMENTS
a(n) = 3 if and only if n is prime.
FORMULA
MATHEMATICA
a[n_] := 2*(DivisorSigma[1, n] - n) + 1; Array[a, 100] (* Amiram Eldar, Mar 30 2024 *)
PROG
(PARI) a(n) = 2*sigma(n) - 2*n + 1; \\ Michel Marcus, Oct 29 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Oct 28 2017
STATUS
approved