a(0) = 2, a(1) = 5, a(n) = 4 * a(n-1) - a(n-2).

Terms

a(0) =2a(1) =5a(2) =18a(3) =67a(4) =250a(5) =933a(6) =3482a(7) =12995a(8) =48498a(9) =180997a(10) =675490a(11) =2520963a(12) =9408362a(13) =35112485a(14) =131041578a(15) =489053827a(16) =1825173730a(17) =6811641093a(18) =25421390642a(19) =94873921475a(20) =354074295258