a(n) = 4^n - 2*2^n + 3.

Terms

a(0) =3a(1) =11a(2) =51a(3) =227a(4) =963a(5) =3971a(6) =16131a(7) =65027a(8) =261123a(9) =1046531a(10) =4190211a(11) =16769027a(12) =67092483a(13) =268402691a(14) =1073676291a(15) =4294836227a(16) =17179607043a(17) =68718952451a(18) =274876858371