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 A123047 #30 Aug 16 2021 21:29:44
%S A123047 1,3,129,60767,76433163,454034821,26675718567,164304369911289
%N A123047 Numbers k that divide 5^k + 4.
%C A123047 k must be odd since any power of 5 plus 4 is odd. - _Robert G. Wilson v_, Nov 14 2006
%C A123047 a(9) > 10^15. - _Max Alekseyev_, Oct 17 2016
%C A123047 Large term (may not be the next one): 3014733401203184049549. - _Max Alekseyev_, Oct 18 2013
%t A123047 Do[ If[ PowerMod[5, 2n - 1, 2n - 1] + 5 == 2n, Print[2n - 1]], {n, 10^9}] (* _Robert G. Wilson v_ *)
%o A123047 (PARI) is(n)=Mod(5,n)^n==-4 \\ _Charles R Greathouse IV_, Nov 04 2016
%Y A123047 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), A125949 (k=4), this sequence (k=-4), A123091 (k=5), A015891 (k=-5), A277350 (k=6), A277348 (k=-6).
%K A123047 hard,more,nonn
%O A123047 1,2
%A A123047 _Alexander Adamchuk_, Nov 04 2006
%E A123047 a(4) and a(5) from _Robert G. Wilson v_, Nov 14 2006
%E A123047 a(7) from _Ryan Propper_, Mar 23 2007
%E A123047 a(8) from _Max Alekseyev_, Oct 17 2016