a(n) = a(n-1) + 4*a(n-2) - 4*a(n-3) for n>3.

Terms

a(0) =1a(1) =2a(2) =4a(3) =7a(4) =15a(5) =27a(6) =59a(7) =107a(8) =235a(9) =427a(10) =939a(11) =1707a(12) =3755a(13) =6827a(14) =15019a(15) =27307a(16) =60075a(17) =109227a(18) =240299a(19) =436907a(20) =961195a(21) =1747627a(22) =3844779a(23) =6990507a(24) =15379115a(25) =27962027a(26) =61516459a(27) =111848107a(28) =246065835a(29) =447392427