4^n+3^n-2^n.

Terms

a(0) =1a(1) =5a(2) =21a(3) =83a(4) =321a(5) =1235a(6) =4761a(7) =18443a(8) =71841a(9) =281315a(10) =1106601a(11) =4369403a(12) =17304561a(13) =68694995a(14) =273202041a(15) =1088057963a(16) =4337948481a(17) =17308878275a(18) =69106635081a(19) =276039644123