A076825 Let u(1)=1, u(n)=2^u(n-1) (mod n), sequence gives values of n such that u(n)=1.

1, 3, 15, 21, 51, 65, 95, 165, 255, 261, 273, 285, 295, 345, 387, 465, 507, 513, 565, 641, 837, 1025, 1353, 1665, 1731, 1845, 1953, 1971, 2013, 2123, 2193, 2231, 2325, 2979, 3087, 3237, 3591, 3615, 3645, 3865, 4097, 4369, 4405, 4445, 4655, 4697, 4797, 4995

Offset

1,2

Links

Paolo Xausa, Table of n, a(n) for n = 1..1000

Formula

Conjecture: a(n)/n^2 -> oo.

Mathematica

Flatten[Position[FoldList[PowerMod[2, #, #2] &, Range[10000]], 1]] (* Paolo Xausa, Jan 19 2026 *)

Crossrefs

Positions of ones in A386588.

Positions of ones in A131644 (starting from 15).

Sequence in context: A147017 A171570 A350987 * A259745 A118565 A230658

Adjacent sequences: A076822 A076823 A076824 * A076826 A076827 A076828

Keyword

nonn

Author

Benoit Cloitre, Nov 24 2002

Extensions

a(1)=1 inserted by Paolo Xausa, Jan 19 2026

Status

approved