A271376 Integers n such that n!/(n-2) + 1 is prime.

3, 4, 5, 6, 7, 10, 12, 13, 21, 24, 37, 64, 68, 136, 169, 216, 276, 435, 617, 753, 1722, 1775, 2762, 3974, 5006, 5931, 7480, 11442, 11896, 13200, 13534, 23240, 27971, 31867

Offset

1,1

Comments

Corresponding primes are 7, 13, 41, 181, 1009, 453601, ...

Links

Table of n, a(n) for n=1..34.

Example

3 is a term because 2*3 + 1 = 7 is prime.
4 is a term because 1*3*4 + 1 = 13 is prime.
5 is a term because 1*2*4*5 + 1 = 41 is prime.
6 is a term because 1*2*3*5*6 + 1 = 181 is prime.
7 is a term because 1*2*3*4*6*7 + 1 = 1009 is prime.

Mathematica

Select[Range[3, 2000], PrimeQ[( #! / (# - 2) + 1)] &] (* Vincenzo Librandi, Apr 07 2016 *)

Prog

(PARI) lista(nn) = for(n=3, nn, if(ispseudoprime(n!/(n-2)+1), print1(n, ", ")));
(PFGW) ABC2 $a!/($a-2) + 1
a: from 3 to 100000
(Magma) [n: n in [3..500] | IsPrime(Factorial(n)  div (n-2) + 1)]; // Vincenzo Librandi, Apr 07 2016

Crossrefs

Cf. A002981, A052747.

Sequence in context: A359772 A158008 A106155 * A087190 A085038 A163078

Adjacent sequences: A271373 A271374 A271375 * A271377 A271378 A271379

Keyword

nonn

Author

Altug Alkan, Apr 05 2016

Extensions

a(23) from Charles R Greathouse IV, Apr 05 2016

a(24)-a(27) from Charles R Greathouse IV, Apr 06 2016

a(28)-a(32) from Charles R Greathouse IV, Apr 18 2016

a(33) from Charles R Greathouse IV, Apr 30 2016

a(34) from Charles R Greathouse IV, May 09 2016

Status

approved