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

Terms

a(0) =2a(1) =2a(2) =10a(3) =18a(4) =58a(5) =130a(6) =362a(7) =882a(8) =2330a(9) =5858a(10) =15178a(11) =38610a(12) =99322a(13) =253762a(14) =651050a(15) =1666098a(16) =4270298a(17) =10934690a(18) =28015882a(19) =71754642a(20) =183818170a(21) =470836738a(22) =1206109418a(23) =3089456370a(24) =7913894042a(25) =20271719522a(26) =51927295690a(27) =133014173778a(28) =340723356538a(29) =872780051650