OFFSET
1,6
FORMULA
a(n) = Sum_{d|n, d < sqrt(n)} (-1)^(d+1) * d^2.
MATHEMATICA
nmax = 70; CoefficientList[Series[Sum[(-1)^(k + 1) k^2 x^(k (k + 1))/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
CROSSREFS
KEYWORD
sign
AUTHOR
Ilya Gutkovskiy, May 20 2024
STATUS
approved