A303937 - OEIS

%I #41 Nov 26 2022 12:36:33

%S 3,5,19,254,266

%N Numbers k such that k!-1 reversed is a prime.

%C If it exists, a(6) > 3000. - _Jon E. Schoenfield_, May 04 2018

%C If it exists, a(6) > 5703. - _J.W.L. (Jan) Eerland_, Aug 08 2022

%C If it exists, a(6) > 10142. - _J.W.L. (Jan) Eerland_, Aug 14 2022

%C If it exists, a(6) > 15000. - _J.W.L. (Jan) Eerland_, Oct 16 2022

%C If it exists, a(6) > 17506. - _J.W.L. (Jan) Eerland_, Nov 26 2022

%e 5 is in the sequence because 5!-1 reversed is 911, which is prime.

%t ParallelTable[If[PrimeQ[IntegerReverse[Factorial[k]-1]],k,Nothing],{k,1,5703}]//.{}->Nothing (* _J.W.L. (Jan) Eerland_, Aug 08 2022 *)

%o (PARI) isok(n) = isprime(fromdigits(Vecrev(digits(n!-1))));

%Y Cf. A298702, A033312.

%K nonn,base,more

%O 1,1

%A _Paolo Galliani_, May 03 2018