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%I #24 Jan 20 2015 02:23:00
%S 2,7,47,383,10321919,51011754393599,
%T 1130138339199322632554990773529330319359999999,
%U 73562883979319395645666688474019139929848516028923903999999999
%N Primes of the form n!! - 1.
%D G. Balzarotti and P. P. Lava, Le sequenze di numeri interi, Hoepli, 2008, p. 158.
%H Vincenzo Librandi, <a href="/A117141/b117141.txt">Table of n, a(n) for n = 1..15</a>
%F a(n) = A093173(n-1) for n>1. - _Alexander Adamchuk_, Apr 18 2007
%e 6!! - 1 = 6*4*2 - 1 = 48 - 1 = 47, which is prime.
%e 8!! - 1 = 8*6*4*2 - 1 = 384 - 1 = 383, which is prime.
%p SFACT:= proc(n) local i,j,k; for k from 1 by 1 to n do i:=k; j:=k-2; while j >0 do i:=i*j; j:=j-2; od: if isprime(i-1) then print(i-1); fi; od: end: SFACT(100);
%t lst={};Do[p=n!!-1;If[PrimeQ[p],AppendTo[lst,p]],{n,0,5!,1}];lst (* _Vladimir Joseph Stephan Orlovsky_, Jan 27 2009 *)
%t Select[Table[n!!-1,{n,1,100}],PrimeQ] (* _Vincenzo Librandi_, Dec 07 2011 *)
%o (PARI) print1(2);for(n=1, 1e3, if(ispseudoprime(t=n!<<n-1), print1(", "t))) \\ _Charles R Greathouse IV_, Jun 16 2011
%Y Cf. A002981, A002982, A088332.
%Y Cf. A093173 = primes of the form (2^n * n!) - 1.
%K nonn
%O 1,1
%A _Paolo P. Lava_ and _Giorgio Balzarotti_, Apr 21 2006
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