a(n) = Sum_{k=0..n} binomial(5*n,k).

Terms

a(0) =1a(1) =6a(2) =56a(3) =576a(4) =6196a(5) =68406a(6) =768212a(7) =8731848a(8) =100146724a(9) =1156626990a(10) =13432735556a(11) =156713948672