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

Terms

a(0) =0a(1) =2a(2) =9a(3) =35a(4) =132a(5) =494a(6) =1845a(7) =6887a(8) =25704a(9) =95930a(10) =358017a(11) =1336139a(12) =4986540a(13) =18610022a(14) =69453549a(15) =259204175a(16) =967363152a(17) =3610248434a(18) =13473630585a(19) =50284273907a(20) =187663465044a(21) =700369586270