a(n) = 2^(n+1) + 3^n + 3.

Terms

a(0) =6a(1) =10a(2) =20a(3) =46a(4) =116a(5) =310a(6) =860a(7) =2446a(8) =7076a(9) =20710a(10) =61100a(11) =181246a(12) =539636a(13) =1610710a(14) =4815740a(15) =14414446a(16) =43177796a(17) =129402310a(18) =387944780a(19) =1163310046a(20) =3488881556a(21) =10464547510a(22) =31389448220a(23) =94159956046a(24) =282463090916a(25) =847355718310