a(n) = 4^n - 2^n - 1.

Terms

a(0) =-1a(1) =1a(2) =11a(3) =55a(4) =239a(5) =991a(6) =4031a(7) =16255a(8) =65279a(9) =261631a(10) =1047551a(11) =4192255a(12) =16773119a(13) =67100671a(14) =268419071a(15) =1073709055a(16) =4294901759a(17) =17179738111a(18) =68719214591a(19) =274877382655