a(0) = 3, a(1) = 8, a(n+2) = 4*a(n+1) - a(n) - 2.

Terms

a(0) =3a(1) =8a(2) =27a(3) =98a(4) =363a(5) =1352a(6) =5043a(7) =18818a(8) =70227a(9) =262088a(10) =978123a(11) =3650402a(12) =13623483a(13) =50843528a(14) =189750627a(15) =708158978a(16) =2642885283a(17) =9863382152a(18) =36810643323a(19) =137379191138a(20) =512706121227