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A303447
Numbers m such that both m and 2m+1 are Fermat pseudoprimes base 2 (A001567).
5
9890881, 23456248059221, 96076792050570581
OFFSET
1,1
COMMENTS
No other terms below 2^64.
Terms a(2) and a(3) are of the form (4^k-1)/3=A002450(k). Terms A002450(k) belong to this sequence for k in A303009. Correspondingly, a(4) <= A002450(A303009(3)) = (4^41-1)/3 = 1611901092819505566274901.
CROSSREFS
Numbers 2*a(n)+1 are listed in A303448.
Subsequence of A006970.
Sequence in context: A233839 A059044 A366414 * A107617 A234980 A179737
KEYWORD
bref,hard,more,nonn
AUTHOR
Max Alekseyev, Apr 24 2018
EXTENSIONS
a(1) from Amiram Eldar, Jan 26 2018
STATUS
approved