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

Terms

a(0) =1a(1) =5a(2) =25a(3) =113a(4) =481a(5) =1985a(6) =8065a(7) =32513a(8) =130561a(9) =523265a(10) =2095105a(11) =8384513a(12) =33546241a(13) =134201345a(14) =536838145a(15) =2147418113a(16) =8589803521a(17) =34359476225a(18) =137438429185a(19) =549754765313