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%I #9 May 13 2021 19:36:25
%S 1,4,10,15,20,65,124,190,217,310,435,561,781,1105,1541,1729,1891,2465,
%T 2821,3565,3820,4123,4495,5461,5611,5662,5731,6601,6735,7449,7813,
%U 8029,8290,8911,9881,10585,11041,11476,12801,13021,13333,13981,14981
%N Nonprimes n such that 5^n==5 (mod n).
%C Theorem: If both numbers q and 2q-1 are primes then n=q*(2q-1) is in the sequence iff q=3 or q is of the form 10k+1. 15,1891,88831,146611,218791,721801,873181,... are such terms.
%H Amiram Eldar, <a href="/A122782/b122782.txt">Table of n, a(n) for n = 1..10000</a>
%t Select[Range[15000], ! PrimeQ[ # ] && Mod[5^#, # ] == Mod[5, # ] &]
%t Join[{1,4},Select[Range[15000],CompositeQ[#]&&PowerMod[5,#,#]==5&]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, May 13 2021 *)
%Y Cf. A005936.
%K nonn
%O 1,2
%A _Farideh Firoozbakht_, Sep 12 2006
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