a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) + a(n-4).

Terms

a(0) =1a(1) =3a(2) =11a(3) =31a(4) =71a(5) =145a(6) =289a(7) =601a(8) =1321a(9) =2979a(10) =6683a(11) =14743a(12) =32111a(13) =69697a(14) =151777a(15) =332113a(16) =728689a(17) =1598883a(18) =3503627a(19) =7668079a(20) =16774775a(21) =36704017a(22) =80343361a(23) =175916521a(24) =385196761a(25) =843365379a(26) =1846290395a(27) =4041672871a(28) =8847607391a(29) =19368919297