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

Terms

a(0) =3a(1) =13a(2) =55a(3) =233a(4) =971a(5) =3981a(6) =16143a(7) =65041a(8) =261139a(9) =1046549a(10) =4190231a(11) =16769049a(12) =67092507a(13) =268402717a(14) =1073676319a(15) =4294836257a(16) =17179607075a(17) =68718952485a(18) =274876858407