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 A125949 #12 Aug 16 2021 21:32:52
%S A125949 1,4769,8563651,300414792131,2353957351049,15960089894129,
%T A125949 452045914836301,657236915690111
%N A125949 Numbers k that divide 5^k - 4.
%C A125949 No other terms below 10^15. - _Max Alekseyev_, Oct 17 2016
%t A125949 a(1) = 1; Do[ If[ PowerMod[5, 2n - 1, 2n - 1] - 4 == 0, Print[2n - 1]], {n,10^9}]
%o A125949 (PARI) is(n)=Mod(5,n)^n==4 \\ _Charles R Greathouse IV_, May 15 2013
%Y A125949 Solutions to 5^n == k (mod n): A067946 (k=1), A015951 (k=-1), A124246 (k=2), A123062 (k=-2), A123061 (k=3), A123052 (k=-3), this sequence (k=4), A123047 (k=-4), A123091 (k=5), A015891 (k=-5), A277350 (k=6), A277348 (k=-6).
%K A125949 hard,more,nonn
%O A125949 1,2
%A A125949 _Alexander Adamchuk_, Feb 04 2007
%E A125949 a(4)-a(8) from _Max Alekseyev_, Jun 09 2010, Oct 17 2016