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

Terms

a(0) =2a(1) =4a(2) =10a(3) =32a(4) =114a(5) =420a(6) =1562a(7) =5824a(8) =21730a(9) =81092a(10) =302634a(11) =1129440a(12) =4215122a(13) =15731044a(14) =58709050a(15) =219105152a(16) =817711554a(17) =3051741060a(18) =11389252682a(19) =42505269664a(20) =158631825970a(21) =592022034212