OFFSET
0,19
LINKS
Eric Weisstein's World of Mathematics, Heptagonal Number
FORMULA
G.f.: Product_{k>=1} (1 + x^(k*(5*k-3)/2))*(1 + x^(k*(5*k+3)/2)).
EXAMPLE
a(18) = 2 because we have [18] and [13, 4, 1].
MATHEMATICA
nmax = 100; CoefficientList[Series[Product[(1 + x^(k (5 k - 3)/2)) (1 + x^(k (5 k + 3)/2)), {k, 1, nmax}], {x, 0, nmax}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Nov 05 2017
STATUS
approved