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A015911 Numbers k such that 2^k mod k is odd.

%I A015911 #24 May 10 2021 11:13:52
%S A015911 25,45,55,91,95,99,125,135,143,153,155,161,175,187,225,235,245,247,
%T A015911 261,273,275,279,285,289,297,319,329,333,335,355,363,369,387,391,403,
%U A015911 407,413,423,425,429,435,437,441,459,473,477,481,483,493,507,517,525,529
%N A015911 Numbers k such that 2^k mod k is odd.
%C A015911 All terms are composite: due to Fermat's little theorem, 2^p == 2 (mod p) when p is prime. - _M. F. Hasler_, May 10 2021
%H A015911 Zak Seidov, <a href="/A015911/b015911.txt">Table of n, a(n) for n = 1..10000</a>
%H A015911 Wikipedia, <a href="https://en.wikipedia.org/wiki/Fermat%27s_little_theorem">Fermat's little theorem</a>.
%p A015911 q:= n-> is(2&^n mod n, odd):
%p A015911 select(q, [$1..1000])[];  # _Alois P. Heinz_, May 10 2021
%t A015911 Select[Range@532, OddQ@PowerMod[2, #, # ] &]
%o A015911 (PARI) is(n)=lift(Mod(2,n)^n)%2 \\ _Charles R Greathouse IV_, May 31 2013
%Y A015911 Cf. A015910, A226221.
%K A015911 nonn
%O A015911 1,1
%A A015911 _Robert G. Wilson v_