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A166850 Numbers n such that reversal(n!-n) is prime. 0


%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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Last modified June 21 19:09 EDT 2021. Contains 345365 sequences. (Running on oeis4.)