OFFSET
1,4
FORMULA
a(n) = Sum_{d|n, d>1} (n/d)^d = Sum_{d|n, d<n} d^(n/d).
a(p) = 1, where p is prime.
a(n) = A055225(n) - n.
MATHEMATICA
nmax = 50; CoefficientList[Series[Sum[k^2 x^(2 k)/(1 - k x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
Table[DivisorSum[n, (n/#)^# &, # > 1 &], {n, 1, 50}]
PROG
(PARI) a(n)={sumdiv(n, d, if(d > 1, (n/d)^d))} \\ Andrew Howroyd, Sep 10 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Sep 10 2019
STATUS
approved