OFFSET
0,10
LINKS
Eric Weisstein's World of Mathematics, Powerful Number
EXAMPLE
a(25) = 3 because we have [25], [16, 9] and [16, 8, 1].
MATHEMATICA
nmax = 100; CoefficientList[Series[(1 + x) Product[1 + Boole[Min@ FactorInteger[k][[All, 2]] > 1] x^k, {k, 2, nmax}], {x, 0, nmax}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, May 12 2017
STATUS
approved