a(n) = 4^n - (n+6)*3^(n-1) + (n+2)*2^(n-1).

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =34a(5) =245a(6) =1436a(7) =7483a(8) =36198a(9) =166545a(10) =739792a(11) =3203783a(12) =13617242a(13) =57072925a(14) =236680068a(15) =973578003a(16) =3979881166a(17) =16191039785a(18) =65622734264a(19) =265197899743