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

Terms

a(0) =1a(1) =0a(2) =3a(3) =6a(4) =9a(5) =72a(6) =63a(7) =486a(8) =1053a(9) =2808a(10) =11907a(11) =22518a(12) =99225a(13) =246888a(14) =755487a(15) =2554902a(16) =6488829a(17) =23112216a(18) =63506835a(19) =198653958a(20) =623336553a(21) =1781565192a(22) =5807475711a(23) =16898655942a(24) =52699192029a(25) =161995971384a(26) =484990399395