OFFSET
0,2
FORMULA
a(n) = Sum_{k=0..n} 3^( n*(n-1)/2 - k*(k-1)/2 ).
G.f. A(x) satisfies: A(x) = 1/(1 - x) + x * A(3*x). - Ilya Gutkovskiy, Jun 06 2020
PROG
(PARI) a(n)=sum(k=0, n, 3^((n-k)*(n+k-1)/2))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Mar 14 2006
STATUS
approved