a(n) = 4*a(n-1) - a(n-2) with a(0) = 0, a(1) = 4.

Terms

a(0) =0a(1) =4a(2) =16a(3) =60a(4) =224a(5) =836a(6) =3120a(7) =11644a(8) =43456a(9) =162180a(10) =605264a(11) =2258876a(12) =8430240a(13) =31462084a(14) =117418096a(15) =438210300a(16) =1635423104a(17) =6103482116a(18) =22778505360a(19) =85010539324a(20) =317263651936