OFFSET
1,4
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
FORMULA
G.f.: Sum_{n>=1} n^2*x^(n^2)/(1-x^n). - Joerg Arndt, Jan 30 2011
EXAMPLE
The divisors of 12 which are <= sqrt(12) are 1,2,3, so a(12) = 1^2 + 2^2 + 3^2 = 14.
MATHEMATICA
a[n_]:=Plus@@(Select[Divisors[n], #^2<=n&]^2)
(* Second program: *)
Table[DivisorSum[n, #^2 &, # <= Sqrt@ n &], {n, 81}] (* Michael De Vlieger, Dec 14 2017 *)
PROG
(PARI) N=100; x='x+O('x^N); Vec( sum(n=1, N, n^2*x^(n^2)/(1-x^n) ) ) \\ Joerg Arndt, Jan 02 2017
(PARI) a(n) = my(rn = sqrt(n)); sumdiv(n, d, d^2*(d<=rn)); \\ Michel Marcus, Jan 02 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Dean Hickerson, following a suggestion of Leroy Quet, May 28 2004
STATUS
approved