Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #12 Aug 16 2021 21:32:52
%S 1,4769,8563651,300414792131,2353957351049,15960089894129,
%T 452045914836301,657236915690111
%N Numbers k that divide 5^k - 4.
%C No other terms below 10^15. - _Max Alekseyev_, Oct 17 2016
%t a(1) = 1; Do[ If[ PowerMod[5, 2n - 1, 2n - 1] - 4 == 0, Print[2n - 1]], {n,10^9}]
%o (PARI) is(n)=Mod(5,n)^n==4 \\ _Charles R Greathouse IV_, May 15 2013
%Y 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 hard,more,nonn
%O 1,2
%A _Alexander Adamchuk_, Feb 04 2007
%E a(4)-a(8) from _Max Alekseyev_, Jun 09 2010, Oct 17 2016