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Lesser of 2 consecutive Fermat pseudoprimes to base 2 with no prime numbers in between them.
1

%I #10 Jul 31 2022 07:46:19

%S 4369,13741,31609,6973057,208969201

%N Lesser of 2 consecutive Fermat pseudoprimes to base 2 with no prime numbers in between them.

%C The corresponding larger counterparts are 4371, 13747, 31621, 6973063, 208969223.

%C The corresponding gaps between the consecutive pseudoprimes are 2, 6, 12, 6, 22.

%C a(6) > 10^15, if it exists.

%e 4369 is a term since it is a Fermat pseudoprime to base 2, and between it and the next Fermat pseudoprime to base 2, 4371, there is only one number, 4370, which is composite.

%t psp = Cases[Import["https://oeis.org/A001567/b001567.txt", "Table"], {_, _}][[;; , 2]]; s = {}; Do[If[NextPrime[psp[[k]]] > psp[[k + 1]], AppendTo[s, psp[[k]]]], {k, 1, Length[psp] - 1}]; s

%Y Cf. A001567, A173619, A175736, A175737, A175738.

%K nonn,hard,more

%O 1,1

%A _Amiram Eldar_, Jun 01 2020