This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A128121 #26 Sep 06 2021 19:13:09 %S A128121 1,3,19147,129505699483,674344345281,1643434407157,5675297754009, %T A128121 12174063716147,162466075477787,313255455573801,324082741109271 %N A128121 Numbers k such that 2^k == 5 (mod k). %H A128121 Joe K. Crump, <a href="https://web.archive.org/web/20120104074313/http://www.immortaltheory.com/NumberTheory/2nmodn.htm">2^n mod n</a> %H A128121 OEIS Wiki, <a href="/wiki/2^n mod n">2^n mod n</a> %t A128121 m = 5; Join[Select[Range[m], Divisible[2^# - m, #] &], %t A128121 Select[Range[m + 1, 10^6], PowerMod[2, #, #] == m &]] (* _Robert Price_, Oct 08 2018 *) %Y A128121 Cf. A015910, A036236, A050259 (numbers k such that 2^k == 3 (mod k)), A033981, A051447, A033982, A051446, A033983, A128122, A128123, A128124, A128125, A128126. %K A128121 hard,more,nonn %O A128121 1,2 %A A128121 _Alexander Adamchuk_, Feb 15 2007 %E A128121 1 and 3 added by _N. J. A. Sloane_, Apr 23 2007 %E A128121 Missing a(10) inserted by _Sergey Paramonov_, Sep 06 2021