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

Terms

a(0) =0a(1) =2a(2) =12a(3) =56a(4) =240a(5) =992a(6) =4032a(7) =16256a(8) =65280a(9) =261632a(10) =1047552a(11) =4192256a(12) =16773120a(13) =67100672a(14) =268419072a(15) =1073709056a(16) =4294901760a(17) =17179738112a(18) =68719214592a(19) =274877382656