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Numbers occurring as divisors of 2^k + 5^k, k > 0.
1

%I #11 Dec 09 2022 15:42:19

%S 1,7,11,17,19,23,29,37,41,47,49,53,59,61,73,77,89,97,101,103,109,113,

%T 121,127,131,133,137,139,149,161,167,173,179,181,193,209,211,223,229,

%U 233,241,251,253,257,259,263,269,277,281,287,289,313,317,329,331,337,343

%N Numbers occurring as divisors of 2^k + 5^k, k > 0.

%C If k is a term, then so are all divisors of k. - _Robert Israel_, Dec 08 2022

%H Robert Israel, <a href="/A346991/b346991.txt">Table of n, a(n) for n = 1..10000</a>

%p filter:= proc(n) local q, v;

%p if igcd(n,10) <> 1 then return false fi;

%p q:= 5/2 mod n;

%p traperror(NumberTheory:-ModularLog(-1,q,n)) <> lasterror

%p end proc:

%p filter(1):= true:

%p select(filter, [$1..400]); # _Robert Israel_, Dec 08 2022

%Y Cf. A074600, A043292.

%K nonn

%O 1,2

%A _Hugo Pfoertner_, Aug 11 2021