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

Terms

a(0) =0a(1) =4a(2) =14a(3) =40a(4) =110a(5) =304a(6) =854a(7) =2440a(8) =7070a(9) =20704a(10) =61094a(11) =181240a(12) =539630a(13) =1610704a(14) =4815734a(15) =14414440a(16) =43177790a(17) =129402304a(18) =387944774a(19) =1163310040a(20) =3488881550a(21) =10464547504a(22) =31389448214a(23) =94159956040a(24) =282463090910a(25) =847355718304