OFFSET
0,5
LINKS
Indranil Ghosh, Table of n, a(n) for n = 0..100
Eric Weisstein's World of Mathematics, Tetrahedral Number
FORMULA
G.f.: 1/(1 - Sum_{k>=1} x^(k*(k+1)*(k+2)/6)).
EXAMPLE
a(8) = 7 because we have [4, 4], [4, 1, 1, 1, 1], [1, 4, 1, 1, 1], [1, 1, 4, 1, 1], [1, 1, 1, 4, 1], [1, 1, 1, 1, 4] and [1, 1, 1, 1, 1, 1, 1, 1].
MATHEMATICA
nmax = 50; CoefficientList[Series[1/(1 - Sum[x^(k (k + 1) (k + 2)/6), {k, 1, nmax}]), {x, 0, nmax}], x]
PROG
(PARI) Vec(1/(1 - sum(k=1, 50, x^(k*(k + 1)*(k + 2)/6)) + O(x^51))) \\ Indranil Ghosh, Mar 15 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Feb 19 2017
STATUS
approved