OFFSET
0,15
LINKS
Eric Weisstein's World of Mathematics, Multiplicative Perfect Number
FORMULA
G.f.: Product_{k>=1} (1 + x^A007422(k)).
EXAMPLE
a(15) = 3 because we have [15], [14, 1] and [8, 6, 1].
MATHEMATICA
nmax = 85; CoefficientList[Series[Product[1 + Boole[Sqrt[k]^DivisorSigma[0, k]/k == k] x^k, {k, 1, nmax}], {x, 0, nmax}], x]
PROG
(PARI) a(k) = k==1 || numdiv(k) == 4;
Vec(prod(k=1, 85, 1 + a(k)*x^k) + O(x^86)) \\ Indranil Ghosh and modified by Michel Marcus, Mar 17 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Mar 16 2017
STATUS
approved