OFFSET
1,2
COMMENTS
Sum of divisors d of n such that n/d is triangular number.
FORMULA
a(n) = Sum_{d|n} A010054(n/d) * d.
MATHEMATICA
nmax = 70; CoefficientList[Series[Sum[x^(k (k + 1)/2)/(1 - x^(k (k + 1)/2))^2, {k, 1, nmax}], {x, 0, nmax}], x] // Rest
a[n_] := DivisorSum[n, # &, IntegerQ[Sqrt[8 n/# + 1]] &]; Table[a[n], {n, 1, 70}]
PROG
(PARI) a(n)={sumdiv(n, d, if(ispolygonal(d, 3), n/d))} \\ Andrew Howroyd, Sep 19 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Sep 19 2019
STATUS
approved