%I #24 May 10 2021 11:13:52

%S 25,45,55,91,95,99,125,135,143,153,155,161,175,187,225,235,245,247,

%T 261,273,275,279,285,289,297,319,329,333,335,355,363,369,387,391,403,

%U 407,413,423,425,429,435,437,441,459,473,477,481,483,493,507,517,525,529

%N Numbers k such that 2^k mod k is odd.

%C 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 Zak Seidov, <a href="/A015911/b015911.txt">Table of n, a(n) for n = 1..10000</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Fermat%27s_little_theorem">Fermat's little theorem</a>.

%p q:= n-> is(2&^n mod n, odd):

%p select(q, [$1..1000])[]; # _Alois P. Heinz_, May 10 2021

%t Select[Range@532, OddQ@PowerMod[2, #, # ] &]

%o (PARI) is(n)=lift(Mod(2,n)^n)%2 \\ _Charles R Greathouse IV_, May 31 2013

%Y Cf. A015910, A226221.

%K nonn

%O 1,1

%A _Robert G. Wilson v_