a(n) = Sum_{k=0..n} binomial(3*n+1,4*k).

Terms

a(0) =1a(1) =2a(2) =36a(3) =256a(4) =2016a(5) =16512a(6) =130816a(7) =1048576a(8) =8390656a(9) =67100672a(10) =536887296a(11) =4294967296a(12) =34359607296a(13) =274878431232