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

Terms

a(0) =0a(1) =10a(2) =54a(3) =238a(4) =990a(5) =4030a(6) =16254a(7) =65278a(8) =261630a(9) =1047550a(10) =4192254a(11) =16773118a(12) =67100670a(13) =268419070a(14) =1073709054a(15) =4294901758a(16) =17179738110a(17) =68719214590a(18) =274877382654