OFFSET
1,1
COMMENTS
It's obvious that there is no number k such that k!-k is prime.
There is no further term up to 5000.
There is no further term up to 25000. - Michael S. Branicky, Nov 14 2024
EXAMPLE
56!-56 =
710998587804863451854045647463724949736497978881168458687447039999999999944
and reversal(56!-56) =
449999999999930744786854861188879794637949427364746540458154368408785899017
is prime, so 56 is in the sequence.
MATHEMATICA
reversal[n_]:=FromDigits[Reverse[IntegerDigits[n]]];
Do[If[PrimeQ[reversal[n!-n]], Print[n]], {n, 5000}]
CROSSREFS
KEYWORD
base,hard,more,nonn
AUTHOR
Farideh Firoozbakht, Oct 22 2009
STATUS
approved