A303447 Numbers m such that both m and 2m+1 are Fermat pseudoprimes base 2 (A001567).

9890881, 23456248059221, 96076792050570581

Offset: 1

Author

Max Alekseyev, Apr 24 2018

Keywords

• bref
• hard
• more
• nonn

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.

Cf. A298758, A300193.

Extensions

a(1) from Amiram Eldar, Jan 26 2018