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#21 by Joerg Arndt at Sat Sep 04 05:53:33 EDT 2021
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#20 by Michel Marcus at Sat Sep 04 05:51:08 EDT 2021
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#19 by Amiram Eldar at Sat Sep 04 04:50:49 EDT 2021
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#18 by Amiram Eldar at Sat Sep 04 04:45:54 EDT 2021
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Composite numbers nk such that sum of proper divisors of nk divides 2^nk-1.
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#17 by Amiram Eldar at Sat Sep 04 04:45:02 EDT 2021
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Amiram Eldar, <a href="/A278315/b278315.txt">Table of n, a(n) for n = 1..130</a>
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approved
editing
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#16 by Harvey P. Dale at Tue Jul 31 11:23:18 EDT 2018
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#15 by Harvey P. Dale at Tue Jul 31 11:23:13 EDT 2018
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Select[Range[8*10^6], CompositeQ[#]&&PowerMod[2, #, DivisorSigma[1, #]-#] == 1&] (* Harvey P. Dale, Jul 31 2018 *)
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approved
editing
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#14 by N. J. A. Sloane at Sat Dec 03 12:32:49 EST 2016
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#13 by Altug Alkan at Sat Nov 19 04:13:45 EST 2016
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#12 by Altug Alkan at Sat Nov 19 04:13:14 EST 2016
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(PARI) is(n)=Mod(2, sigma(n)-n)^n==1; && !isprime(n);
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reviewed
editing
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Discussion
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| Sat Nov 19
| 04:13
| Altug Alkan: Thank you very much, sorry, best regards.
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