|
%I #12 Jan 17 2023 15:11:15
%S 5,911,999138804001546121
%N Primes of the form R(n!-1), where R is the digit reversal.
%C The next term is too large to include.
%C Values of n are 3, 5, 19, 254, 266, ... [_Charles R Greathouse IV_, Feb 15 2011]
%C The next term (a(4)) has 503 digits. - _Harvey P. Dale_, Jan 17 2023
%e n=5 -> 5!-1 = 120-1=119 -> R(119)=911 that is prime
%p P:=proc(i) local a,b,k,n,v; v:=array(1..10000); for n from 1 by 1 to i do a:=1; k:=n!-1; while k>0 do v[a]:=k-(trunc(k/10)*10); k:=trunc(k/10); a:=a+1; od; k:=0; for b from a-1 by -1 to 1 do k:=k+v[b]*10^(a-1-b); od; if isprime(k) then print(k); fi; od; end: P(2000);
%t Select[IntegerReverse/@(Range[20]!-1),PrimeQ] (* _Harvey P. Dale_, Jan 17 2023 *)
%Y Cf. A173913, A173915, A173916.
%K nonn,bref,base
%O 1,1
%A _Paolo P. Lava_ and _Giorgio Balzarotti_, Mar 02 2010
| 