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A123047 Numbers n such that n divides 5^n + 4. 9
1, 3, 129, 60767, 76433163, 454034821, 26675718567, 164304369911289 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

n 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 June 20 16:26 EDT 2019. Contains 324234 sequences. (Running on oeis4.)