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%I #14 Apr 24 2018 21:32:21
%S 19781763,46912496118443,192153584101141163
%N Numbers m such that both m and (m-1)/2 are Fermat pseudoprimes base 2 (A001567).
%C No other terms below 2^65.
%C Terms a(2) and a(3) are of the form (2^(2k+1)+1)/3 = A007583(k).
%C Terms A007583(k) belong to this sequence for k in A303009. Correspondingly, a(4) <= A007583(A303009(3)) = (2^83+1)/3 = 3223802185639011132549803.
%C If a(n) is not divisible by 3, then it also belongs to A175625.
%F a(n) = 2*A303447(n) + 1.
%Y Numbers (a(n)-1)/2 are listed in A303447.
%Y Subsequence of A006970 and A300193.
%Y Cf. A175625, A175942, A298758.
%K bref,hard,more,nonn
%O 1,1
%A _Max Alekseyev_, Apr 24 2018
%E a(1) from _Amiram Eldar_, Jan 26 2018
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