Start with 1; square; add 2; subtract 1; repeat.

Terms

a(0) =1a(1) =1a(2) =3a(3) =2a(4) =4a(5) =6a(6) =5a(7) =25a(8) =27a(9) =26a(10) =676a(11) =678a(12) =677a(13) =458329a(14) =458331a(15) =458330a(16) =210066388900a(17) =210066388902a(18) =210066388901