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%I #12 Oct 25 2018 19:33:35
%S 5,9,10,14,15,16,17,18,19,20,21,22,23,25,26,27,28,29,30,31,32,33,34,
%T 35,36,37,38,39,40,41,42,43,44,45,46,48,50,51,52,53,54,55,56,57,58,59,
%U 60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80
%N Numbers k such that 5^k-1 contains a divisor which is an overpseudoprime to base 5.
%C An odd prime p is in the sequence iff p is not in A004061.
%o (PARI) isokd(n) = (n>5) && !isprime(n) && (gcd(n,5)==1) && (znorder(Mod(5,n)) * (sumdiv(n, d, eulerphi(d)/znorder(Mod(5, d))) - 1) + 1 == n); \\ A141390
%o isok(n) = {fordiv (5^n-1, dd, if (isokd(dd), return (1));); return (0);} \\ _Michel Marcus_, Oct 25 2018
%Y Cf. A141390, A020231, A141232, A141350, A003463, A004061.
%K nonn
%O 1,1
%A _Vladimir Shevelev_, Jun 30 2008
%E Corrected and more terms from _Michel Marcus_, Oct 25 2018
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