Let u(1)=1, u(n)=2^u(n-1) (mod n), sequence gives values of n such that u(n)=1.

Terms

a(0) =1a(1) =3a(2) =15a(3) =21a(4) =51a(5) =65a(6) =95a(7) =165a(8) =255a(9) =261a(10) =273a(11) =285a(12) =295a(13) =345a(14) =387a(15) =465a(16) =507a(17) =513a(18) =565a(19) =641a(20) =837a(21) =1025a(22) =1353a(23) =1665a(24) =1731a(25) =1845a(26) =1953a(27) =1971a(28) =2013a(29) =2123