OFFSET
21,1
LINKS
Alois P. Heinz, Table of n, a(n) for n = 21..1000
FORMULA
a(n) = n! * [x^n*y^6] Product_{n>=1} (1+y*x^n/n!).
MAPLE
b:= proc(n, i, t) option remember; `if`(t>i or t*(t+1)/2>n
or t*(2*i+1-t)/2<n, 0, `if`(n=0, 1, b(n, i-1, t)+
`if`(i>n, 0, b(n-i, i-1, t-1)*binomial(n, i))))
end:
a:= n-> b(n$2, 6):
seq(a(n), n=21..40);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, May 01 2016
STATUS
editing