a(n) = -(u^n-1)*(v^n-1) with u = 2+sqrt(3), v = 2-sqrt(3).

Terms

a(0) =2a(1) =12a(2) =50a(3) =192a(4) =722a(5) =2700a(6) =10082a(7) =37632a(8) =140450a(9) =524172a(10) =1956242a(11) =7300800a(12) =27246962a(13) =101687052a(14) =379501250a(15) =1416317952a(16) =5285770562a(17) =19726764300a(18) =73621286642a(19) =274758382272