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

Terms

a(0) =5a(1) =7a(2) =9a(3) =7a(4) =-15a(5) =-113a(6) =-471a(7) =-1673a(8) =-5535a(9) =-17633a(10) =-54951a(11) =-168953a(12) =-515055a(13) =-1561553a(14) =-4717431a(15) =-14217833a(16) =-42784575a(17) =-128615873a(18) =-386371911a(19) =-1160164313a(20) =-3482590095a(21) =-10451964593a(22) =-31364282391a(23) =-94109624393a(24) =-282362427615a(25) =-847154391713