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 A173916 Primes of the form R(n!!-1), where R(k) is the digit reversal of k. 3

%I #5 Aug 24 2012 10:50:03

%S 2,7,41,383,401,449,9911985173,422341432613,9991380634172583438761,

%T 426098774309076039404423265983175582575572211,

%U 4260466555914046005704099160490768769763030425914059033564091456450512175875462052616893583

%N Primes of the form R(n!!-1), where R(k) is the digit reversal of k.

%e n=7 -> 7!!-1 = 105-1=104 -> R(104)=401 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:=doublefactorial(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);

%Y Cf. A173913-A173915

%K nonn,base

%O 1,1

%A _Paolo P. Lava_ and _Giorgio Balzarotti_, Mar 02 2010, Mar 05 2010

%E Edited by _Charles R Greathouse IV_, Aug 02 2010

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Last modified May 28 04:05 EDT 2023. Contains 362992 sequences. (Running on oeis4.)