OFFSET
0,8
LINKS
Eric Weisstein's World of Mathematics, Pentagonal Number
FORMULA
G.f.: Product_{k>=1} (1 + x^(k*(3*k-1)/2))*(1 + x^(k*(3*k+1)/2)).
EXAMPLE
a(15) = 3 because we have [15], [12, 2, 1] and [7, 5, 2, 1].
MATHEMATICA
nmax = 90; CoefficientList[Series[Product[(1 + x^(k (3 k - 1)/2)) (1 + x^(k (3 k + 1)/2)), {k, 1, nmax}], {x, 0, nmax}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Aug 14 2017
STATUS
approved