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

Terms

a(0) =2a(1) =4a(2) =14a(3) =58a(4) =242a(5) =994a(6) =4034a(7) =16258a(8) =65282a(9) =261634a(10) =1047554a(11) =4192258a(12) =16773122a(13) =67100674a(14) =268419074a(15) =1073709058a(16) =4294901762a(17) =17179738114a(18) =68719214594a(19) =274877382658