OFFSET
0,3
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 0..730 (terms 0..90 from Ilya Gutkovskiy)
Eric Weisstein's World of Mathematics, Partition, Partition Function P
Eric Weisstein's World of Mathematics, Pentagonal Number
FORMULA
a(n) ~ exp((Pi*sqrt(n*(3*n - 1)))/sqrt(3))/(2*sqrt(3)*n*(3*n - 1)).
a(n) = [x^(n*(3*n-1)/2)] Product_{k>=1} 1/(1 - x^k). - Ilya Gutkovskiy, Apr 11 2017
EXAMPLE
a(2) = 7, because second pentagonal number is a 5 and 5 can be partitioned in 7 distinct ways: 5, 4 + 1, 3 + 2, 3 + 1 + 1, 3 + 2 + 1, 2 + 1 + 1 + 1, 1 + 1 + 1 + 1 + 1.
MATHEMATICA
Table[PartitionsP[n ((3 n - 1)/2)], {n, 0, 19}]
PROG
(PARI) a(n)=numbpart(n*(3*n-1)/2) \\ Charles R Greathouse IV, Jul 26 2016
(Python)
from sympy.ntheory import npartitions
print([npartitions(n*(3*n - 1)//2) for n in range(51)]) # Indranil Ghosh, Apr 11 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Apr 07 2016
STATUS
approved