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

Terms

a(0) =0a(1) =2a(2) =8a(3) =30a(4) =112a(5) =418a(6) =1560a(7) =5822a(8) =21728a(9) =81090a(10) =302632a(11) =1129438a(12) =4215120a(13) =15731042a(14) =58709048a(15) =219105150a(16) =817711552a(17) =3051741058a(18) =11389252680a(19) =42505269662a(20) =158631825968a(21) =592022034210