Exchange successive pairs of terms of A000051.

Terms

a(0) =3a(1) =2a(2) =9a(3) =5a(4) =33a(5) =17a(6) =129a(7) =65a(8) =513a(9) =257a(10) =2049a(11) =1025a(12) =8193a(13) =4097a(14) =32769a(15) =16385a(16) =131073a(17) =65537a(18) =524289a(19) =262145a(20) =2097153a(21) =1048577a(22) =8388609a(23) =4194305a(24) =33554433a(25) =16777217a(26) =134217729a(27) =67108865a(28) =536870913a(29) =268435457