OFFSET
1,8
COMMENTS
a(n) = 0 iff n is in A138620.
First occurrence of k > -1: 1, 3, 8, 13, 18, 31, 28, 33, 23, 43, 66, 53, 45, 63, 48, 101, 166, etc.
FORMULA
Sum_{k=1..n} a(k) = c * n^2 + O(n*log(n)), where c = Pi^2/18 - 1/2 = 0.048311... . - Amiram Eldar, Mar 28 2024
EXAMPLE
a(13) = 3 since d(12*13-1)/12 - 13 = 192/12 - 13 = 16 - 13 = 3.
MATHEMATICA
a[n_] := DivisorSigma[1, 12 n - 1]/12 - n; Array[a, 90]
PROG
(PARI) a(n) = sigma(12*n-1)/12 - n;
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Omar E. Pol and Robert G. Wilson v, Nov 04 2017
STATUS
approved