OFFSET
0,11
FORMULA
G.f.: Sum_{k>=0} k! * x^(k*(2*k + 1)) / Product_{j=1..k} (1 - x^(4*j)).
EXAMPLE
a(21) = 6 because we have [11, 7, 3], [11, 3, 7], [7, 11, 3], [7, 3, 11], [3, 11, 7] and [3, 7, 11].
MATHEMATICA
nmax = 75; CoefficientList[Series[Sum[k! x^(k (2 k + 1))/Product[1 - x^(4 j), {j, 1, k}], {k, 0, nmax}], {x, 0, nmax}], x]
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Nov 22 2020
STATUS
approved