a(n) = 2*a(n-1) if n odd else 2*a(n-1) + 6.

Terms

a(0) =0a(1) =6a(2) =12a(3) =30a(4) =60a(5) =126a(6) =252a(7) =510a(8) =1020a(9) =2046a(10) =4092a(11) =8190a(12) =16380a(13) =32766a(14) =65532a(15) =131070a(16) =262140a(17) =524286a(18) =1048572a(19) =2097150a(20) =4194300a(21) =8388606a(22) =16777212a(23) =33554430a(24) =67108860a(25) =134217726a(26) =268435452a(27) =536870910