Related to random walks on the 4-cube.

Terms

a(0) =1a(1) =1a(2) =5a(3) =17a(4) =65a(5) =257a(6) =1025a(7) =4097a(8) =16385a(9) =65537a(10) =262145a(11) =1048577a(12) =4194305a(13) =16777217a(14) =67108865a(15) =268435457a(16) =1073741825a(17) =4294967297a(18) =17179869185a(19) =68719476737a(20) =274877906945