a(n) = 2^(2*n+1) - 5*2^(n-1) - 1.

Terms

a(0) =2a(1) =21a(2) =107a(3) =471a(4) =1967a(5) =8031a(6) =32447a(7) =130431a(8) =523007a(9) =2094591a(10) =8383487a(11) =33544191a(12) =134197247a(13) =536829951a(14) =2147401727a(15) =8589770751a(16) =34359410687a(17) =137438298111a(18) =549754503167