OFFSET
1,4
COMMENTS
The sum of the cubes of the divisors of n which are <= sqrt(n).
FORMULA
G.f.: Sum_{k>=1} k^3*x^(k^2)/(1 - x^k).
EXAMPLE
The divisors of 12 which are <= sqrt(12) are {1,2,3}, so a(12) = 1^3 + 2^3 + 3^3 = 36.
MATHEMATICA
nmax = 85; Rest[CoefficientList[Series[Sum[k^3 x^k^2/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x]]
(* Second program *)
Table[Total[Select[Divisors@ n, # <= Sqrt@ n &]^3], {n, 85}] (* Michael De Vlieger, Jan 01 2017 *)
PROG
(PARI) a(n) = my(rn = sqrt(n)); sumdiv(n, d, d^3*(d<=rn)); \\ Michel Marcus, Jan 02 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Jan 01 2017
STATUS
approved