a(n) = 2^(n-1)*(2^n - 1), n >= 0.

Terms

a(0) =0a(1) =1a(2) =6a(3) =28a(4) =120a(5) =496a(6) =2016a(7) =8128a(8) =32640a(9) =130816a(10) =523776a(11) =2096128a(12) =8386560a(13) =33550336a(14) =134209536a(15) =536854528a(16) =2147450880a(17) =8589869056a(18) =34359607296a(19) =137438691328a(20) =549755289600