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A126782 Primes of the form [n! mod (n!!+1)]/2, with n>=1. 0
3, 17, 29, 281, 254993, 690953, 607435538171963, 192133794380608031505991200873083839505054136751452696277424837839455632569607117048950195313 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

Table of n, a(n) for n=1..8.

EXAMPLE

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

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 *)

CROSSREFS

Cf. A055490.

Sequence in context: A107158 A188620 A152529 * A090648 A031024 A045437

Adjacent sequences:  A126779 A126780 A126781 * A126783 A126784 A126785

KEYWORD

hard,nonn

AUTHOR

Paolo P. Lava and Giorgio Balzarotti, Mar 14 2007

STATUS

approved

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Last modified October 28 04:33 EDT 2021. Contains 348313 sequences. (Running on oeis4.)