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%I #15 Sep 10 2019 04:52:25
%S 341,561,645,1105,1729,1905,2465,2701,2821,3277,4033,4369,4681,5461,
%T 6601,7957,8321,8481,10261,10585,11305,12801,13741,13981,15709,15841,
%U 16705,18705,18721,23001,23377,25761,29341,30121,30889,31417,31609,31621,33153,34945,35333,39865,41041,41665,46657,49141,49981,52633
%N Pseudoprimes to base 2 of the form 4k+1.
%H Amiram Eldar, <a href="/A178723/b178723.txt">Table of n, a(n) for n = 1..10000</a>
%t Select[4 * Range[10^4] + 1, CompositeQ[#] && PowerMod[2, # - 1, #] == 1 &] (* _Amiram Eldar_, Sep 10 2019 *)
%o (PARI) forstep(n=5, 10^7, 4, if(isprime(n), next()); if(Mod(2, n)^(n-1)==1, print1(n, ", ")))
%Y Cf. A001567, A177884.
%K nonn
%O 1,1
%A _Joerg Arndt_, Dec 30 2010