a(n) = (5*4^n - 8)/6.

Terms

a(0) =2a(1) =12a(2) =52a(3) =212a(4) =852a(5) =3412a(6) =13652a(7) =54612a(8) =218452a(9) =873812a(10) =3495252a(11) =13981012a(12) =55924052a(13) =223696212a(14) =894784852a(15) =3579139412a(16) =14316557652a(17) =57266230612a(18) =229064922452a(19) =916259689812