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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
OFFSET
1,1
COMMENTS
The next term has 611 digits. - Harvey P. Dale, Apr 15 2018
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 A365234
KEYWORD
hard,nonn
AUTHOR
STATUS
approved