a(n) = 2^n - 2.

Terms

a(0) =-1a(1) =0a(2) =2a(3) =6a(4) =14a(5) =30a(6) =62a(7) =126a(8) =254a(9) =510a(10) =1022a(11) =2046a(12) =4094a(13) =8190a(14) =16382a(15) =32766a(16) =65534a(17) =131070a(18) =262142a(19) =524286a(20) =1048574a(21) =2097150a(22) =4194302a(23) =8388606a(24) =16777214a(25) =33554430a(26) =67108862a(27) =134217726a(28) =268435454a(29) =536870910