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A296243 Numbers k such that the multiplicative order of 2 modulo k is even.

%I A296243 #19 Jul 30 2020 03:57:19
%S A296243 3,5,9,11,13,15,17,19,21,25,27,29,33,35,37,39,41,43,45,51,53,55,57,59,
%T A296243 61,63,65,67,69,75,77,81,83,85,87,91,93,95,97,99,101,105,107,109,111,
%U A296243 113,115,117,119,121,123,125,129,131,133,135,137,139,141,143,145,147,149,153
%N A296243 Numbers k such that the multiplicative order of 2 modulo k is even.
%C A296243 Odd numbers k such that A007733(k) = A002326((k-1)/2) is even.
%H A296243 Amiram Eldar, <a href="/A296243/b296243.txt">Table of n, a(n) for n = 1..10000</a>
%t A296243 A036259 = Select[Range[1, 199, 2], OddQ[MultiplicativeOrder[2, #]] &];
%t A296243 Range[1, A036259[[-1]], 2] ~Complement~ A036259 (* _Jean-François Alcover_, Dec 20 2017 *)
%t A296243 Select[Range[1, 153, 2], EvenQ[MultiplicativeOrder[2, #]] &] (* _Amiram Eldar_, Jul 30 2020 *)
%o A296243 (PARI) { is_A296243(n) = (n%2) && !(znorder(Mod(2,n))%2); }
%Y A296243 Set difference of A005408 and A036259.
%Y A296243 Contains A296244 as a subsequence.
%Y A296243 The prime terms are given by A014662.
%Y A296243 Cf. A002326, A007733.
%K A296243 nonn
%O A296243 1,1
%A A296243 _Max Alekseyev_, Dec 09 2017