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%I #2 Mar 31 2012 14:45:48
%S 3,4,56,128,197,331
%N Numbers n such that reversal(n!-n) is prime.
%C It's obvious that there is no number n such that n!-n is prime.
%C There is no further term up to 5000.
%e 56!-56=
%e 710998587804863451854045647463724949736497978881168458687447039999999999944
%e and reversal(56!-56)=
%e 449999999999930744786854861188879794637949427364746540458154368408785899017
%e is prime so 56 is in the sequence.
%t reversal[n_]:=FromDigits[Reverse[IntegerDigits[n]]];
%t Do[If[PrimeQ[reversal[n!-n]],Print[n]],{n,5000}]
%K base,hard,more,nonn
%O 1,1
%A _Farideh Firoozbakht_, Oct 22 2009
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