OFFSET
1,4
LINKS
Chai Wah Wu, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = [x^(n*(n+1)/2)] Sum_{i>=1} Sum_{j>=i} Product_{k=i..j} x^(k*(k+1)/2).
EXAMPLE
A000217(4) = 10 = 1 + 3 + 6, so a(4) = 2.
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Apr 18 2019
STATUS
approved