a(n) = (5^n - 3)/2.

Terms

a(0) =-1a(1) =1a(2) =11a(3) =61a(4) =311a(5) =1561a(6) =7811a(7) =39061a(8) =195311a(9) =976561a(10) =4882811a(11) =24414061a(12) =122070311a(13) =610351561a(14) =3051757811a(15) =15258789061a(16) =76293945311a(17) =381469726561