a(n) = 4*a(n-1) - a(n-2), with a(0) = 0, a(1) = 3.

Terms

a(0) =0a(1) =3a(2) =12a(3) =45a(4) =168a(5) =627a(6) =2340a(7) =8733a(8) =32592a(9) =121635a(10) =453948a(11) =1694157a(12) =6322680a(13) =23596563a(14) =88063572a(15) =328657725a(16) =1226567328a(17) =4577611587a(18) =17083879020a(19) =63757904493a(20) =237947738952a(21) =888033051315