0,2

a(n) = Sum_{k=0..n} 3^( n*(n-1)/2 - k*(k-1)/2 ).

(PARI) a(n)=sum(k=0, n, 3^((n-k)*(n+k-1)/2))

Adjacent sequences: A117260 A117261 A117262 this_sequence A117264 A117265 A117266

Sequence in context: A100523 A006506 A011821 this_sequence A046855 A116985 A042051

nonn

Paul D. Hanna (pauldhanna(AT)juno.com), Mar 14 2006