A126782 Primes of the form [n! mod (n!!+1)]/2, with n>=1.

3, 17, 29, 281, 254993, 690953, 607435538171963, 192133794380608031505991200873083839505054136751452696277424837839455632569607117048950195313

Offset: 1

Author

Paolo P. Lava and Giorgio Balzarotti, Mar 14 2007

Keywords

• hard
• nonn

Comments

The next term has 611 digits. - Harvey P. Dale, Apr 15 2018

Examples

			n=6 n!=720 n!!=48 [n! mod (n!!+1)]/2 = (720 mod 49)/2 = 34/2 = 17
n=7 n!=5040 n!!=105 [n! mod (n!!+1)]/2 = (5040 mod 106)/2 = 58/2 = 29
		

Crossrefs

Cf. A055490.

Programs

Maple

P:=proc(n) local i,j,k,w; for i from 2 by 1 to n do k:=i; w:=i-2; while w>0 do k:=k*w; w:=w-2; od; j:=(i! mod (k+1))/2; if isprime(j) then print(j); fi; od; end: P(1000);

Mathematica

Select[Table[Mod[n!,n!!+1]/2,{n,200}],PrimeQ] (* Harvey P. Dale, Apr 15 2018 *)