a(n) = 2^(2*n+1) - 2.

Terms

a(0) =0a(1) =6a(2) =30a(3) =126a(4) =510a(5) =2046a(6) =8190a(7) =32766a(8) =131070a(9) =524286a(10) =2097150a(11) =8388606a(12) =33554430a(13) =134217726a(14) =536870910a(15) =2147483646a(16) =8589934590a(17) =34359738366a(18) =137438953470a(19) =549755813886