OFFSET
1,81
COMMENTS
Number of ways to write n as the difference of two heptagonal numbers.
FORMULA
G.f.: Sum_{i>=0} Sum_{j>=i} Product_{k=i..j} x^(5*k + 1).
MATHEMATICA
nmax = 93; CoefficientList[Series[Sum[x^(k (5 k - 3)/2)/(1 - x^(5 k)), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Apr 06 2020
STATUS
approved