a(n) = (3^n - 1)^2*(3^n + 1)/16.

A152257

a(n) = (3^n - 1)^2*(3^n + 1)/16.

Terms

    a(0) =0a(1) =1a(2) =40a(3) =1183a(4) =32800a(5) =893101a(6) =24180520a(7) =653473003a(8) =17649155200a(9) =476575627801

External references