OFFSET
1,4
COMMENTS
The numbers n < 1000 such that n divides a(n) are 4, 10, 42, and 90. (See A224488).
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..27144 (first 1000 terms from Jaroslav Krizek)
FORMULA
G.f.: sum(n>=1, A000203(n)*x^(2*n)/(1-x^n) ). - Mircea Merca, Feb 26 2014
a(n) = Sum_{d|n} A001065(d). - Antti Karttunen, Nov 13 2017
Sum_{k=1..n} a(k) ~ c * n^2, where c = Pi^4/72 - Pi^2/12 = 0.530437... . - Amiram Eldar, Mar 17 2024
MATHEMATICA
Table[Sum[DivisorSigma[1, d], {d, Most[Divisors[n]]}], {n, 100}] (* T. D. Noe, Apr 26 2012 *)
PROG
(PARI) a(n)=sumdiv(n, d, sigma(d))-sigma(n) \\ Charles R Greathouse IV, Feb 19 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Apr 20 2012
STATUS
approved