A128121 Numbers k such that 2^k == 5 (mod k).

1, 3, 19147, 129505699483, 674344345281, 1643434407157, 5675297754009, 12174063716147, 162466075477787, 313255455573801, 324082741109271

Offset: 1

Author

Alexander Adamchuk, Feb 15 2007

Keywords

• hard
• more
• nonn

Links

• Joe K. Crump, 2^n mod n
• OEIS Wiki, 2^n mod n

Crossrefs

Cf. A015910, A036236, A050259 (numbers k such that 2^k == 3 (mod k)), A033981, A051447, A033982, A051446, A033983, A128122, A128123, A128124, A128125, A128126.

Programs

Mathematica

m = 5; Join[Select[Range[m], Divisible[2^# - m, #] &],
Select[Range[m + 1, 10^6], PowerMod[2, #, #] == m &]] (* Robert Price, Oct 08 2018 *)

Extensions

1 and 3 added by N. J. A. Sloane, Apr 23 2007

Missing a(10) inserted by Sergey Paramonov, Sep 06 2021