A104381 Numbers k such that 10^(k-1) == 1 (mod k).

1, 3, 7, 9, 11, 13, 17, 19, 23, 29, 31, 33, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 91, 97, 99, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 259, 263

Offset: 1

Author

Eric W. Weisstein, Mar 03 2005

Keywords

• nonn

Comments

Superset of the full reptend primes.

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000
• Eric Weisstein's World of Mathematics, Full Reptend Prime.

Crossrefs

Cf. A005939.

Programs

Mathematica

Select[ Range[270], Mod[ PowerMod[10, # - 1, # ] - 1, # ] == 0 &] (* Robert G. Wilson v, Mar 16 2005 *)
Join[{1},Select[Range[300],PowerMod[10,#-1,#]==1&]] (* Harvey P. Dale, Oct 24 2016 *)

PARI

isok(n) = lift(Mod(10, n)^(n-1)) == 1; \\ Michel Marcus, Sep 25 2014