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 A123047 Numbers k that divide 5^k + 4. 16
 1, 3, 129, 60767, 76433163, 454034821, 26675718567, 164304369911289 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS k must be odd since any power of 5 plus 4 is odd. - Robert G. Wilson v, Nov 14 2006 a(9) > 10^15. - Max Alekseyev, Oct 17 2016 Large term (may not be the next one): 3014733401203184049549. - Max Alekseyev, Oct 18 2013 LINKS Table of n, a(n) for n=1..8. MATHEMATICA Do[ If[ PowerMod[5, 2n - 1, 2n - 1] + 5 == 2n, Print[2n - 1]], {n, 10^9}] (* Robert G. Wilson v *) PROG (PARI) is(n)=Mod(5, n)^n==-4 \\ Charles R Greathouse IV, Nov 04 2016 CROSSREFS 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). Sequence in context: A300543 A300970 A300927 * A097420 A037119 A113558 Adjacent sequences: A123044 A123045 A123046 * A123048 A123049 A123050 KEYWORD hard,more,nonn AUTHOR Alexander Adamchuk, Nov 04 2006 EXTENSIONS a(4) and a(5) from Robert G. Wilson v, Nov 14 2006 a(7) from Ryan Propper, Mar 23 2007 a(8) from Max Alekseyev, Oct 17 2016 STATUS approved

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Last modified July 20 20:25 EDT 2024. Contains 374459 sequences. (Running on oeis4.)