a(n) = (5^n - 4^n + 3^n - 2^n)/2.

Terms

a(0) =0a(1) =1a(2) =7a(3) =40a(4) =217a(5) =1156a(6) =6097a(7) =31900a(8) =165697a(9) =855076a(10) =4387537a(11) =22404460a(12) =113945377a(13) =577590196a(14) =2919923377a(15) =14729076220a(16) =74167952257a(17) =372944296516