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

Terms

a(0) =2a(1) =3a(2) =13a(3) =59a(4) =249a(5) =1015a(6) =4085a(7) =16371a(8) =65521a(9) =262127a(10) =1048557a(11) =4194283a(12) =16777193a(13) =67108839a(14) =268435429a(15) =1073741795a(16) =4294967265a(17) =17179869151a(18) =68719476701a(19) =274877906907