a(n) = ((4+sqrt(3))*(1+sqrt(3))^n + (4-sqrt(3))*(1-sqrt(3))^n)/2.

Terms

a(0) =4a(1) =7a(2) =22a(3) =58a(4) =160a(5) =436a(6) =1192a(7) =3256a(8) =8896a(9) =24304a(10) =66400a(11) =181408a(12) =495616a(13) =1354048a(14) =3699328a(15) =10106752a(16) =27612160a(17) =75437824a(18) =206099968a(19) =563075584a(20) =1538351104a(21) =4202853376a(22) =11482408960a(23) =31370524672a(24) =85705867264a(25) =234152783872a(26) =639717302272