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

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =17a(5) =37a(6) =69a(7) =149a(8) =277a(9) =597a(10) =1109a(11) =2389a(12) =4437a(13) =9557a(14) =17749a(15) =38229a(16) =70997a(17) =152917a(18) =283989a(19) =611669a(20) =1135957a(21) =2446677a(22) =4543829a(23) =9786709a(24) =18175317a(25) =39146837a(26) =72701269a(27) =156587349a(28) =290805077a(29) =626349397