a(0)=1, a(1)=4; thereafter a(n) = 13*4^n/8-2^(n+1)+1.

Terms

a(0) =1a(1) =4a(2) =19a(3) =89a(4) =385a(5) =1601a(6) =6529a(7) =26369a(8) =105985a(9) =424961a(10) =1701889a(11) =6811649a(12) =27254785a(13) =109035521a(14) =436174849a(15) =1744764929a(16) =6979190785a(17) =27917025281a(18) =111668625409a(19) =446675550209