a(n) = 2*a(n-1) + 2*a(n-2); a(0)=0, a(1)=3.

Terms

a(0) =0a(1) =3a(2) =6a(3) =18a(4) =48a(5) =132a(6) =360a(7) =984a(8) =2688a(9) =7344a(10) =20064a(11) =54816a(12) =149760a(13) =409152a(14) =1117824a(15) =3053952a(16) =8343552a(17) =22795008a(18) =62277120a(19) =170144256a(20) =464842752a(21) =1269974016a(22) =3469633536a(23) =9479215104a(24) =25897697280a(25) =70753824768a(26) =193303044096a(27) =528113737728