

A002981


Numbers k such that k! + 1 is prime.
(Formerly M0908)


108



0, 1, 2, 3, 11, 27, 37, 41, 73, 77, 116, 154, 320, 340, 399, 427, 872, 1477, 6380, 26951, 110059, 150209
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OFFSET

1,3


COMMENTS

If n + 1 is prime then (by Wilson's theorem) n + 1 divides n! + 1. Thus for n > 2 if n + 1 is prime n is not in the sequence.  Farideh Firoozbakht, Aug 22 2003
For n > 2, n! + 1 is prime <==> nextprime((n+1)!) > (n+1)nextprime(n!) and we can conjecture that for n > 2 if n! + 1 is prime then (n+1)! + 1 is not prime.  Mohammed Bouayoun (bouyao(AT)wanadoo.fr), Mar 03 2004
The prime members are in A093804 (numbers n such that Sum_{dn} d! is prime) since Sum_{dn} d! = n! + 1 if n is prime.  Jonathan Sondow
150209 is also in the sequence, cf. the link to Caldwell's prime pages.  M. F. Hasler, Nov 04 2011


REFERENCES

J.M. De Koninck, Ces nombres qui nous fascinent, Entry 116, p. 40, Ellipses, Paris 2008.
Harvey Dubner, Factorial and primorial primes, J. Rec. Math., 19 (No. 3, 1987), 197203.
Richard K. Guy, Unsolved Problems in Number Theory, Section A2.
F. Le Lionnais, Les Nombres Remarquables, Paris, Hermann, 1983, p. 100.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).


LINKS

Table of n, a(n) for n=1..22.
A. Borning, Some results for k!+1 and 2.3.5...p+1, Math. Comp., 26 (1972), 567570.
Chris K. Caldwell, Factorial Primes
Chris K. Caldwell, 110059! + 1 on the Prime Pages
Chris K. Caldwell, 150209! + 1 on the Prime Pages (Nov 03 2011).
Chris K. Caldwell and Y. Gallot, On the primality of n!+1 and 2*3*5*...*p+1, Math. Comp., 71 (2001), 441448.
H. Dubner, Factorial and primorial primes, J. Rec. Math., 19 (No. 3, 1987), 197203. (Annotated scanned copy)
H. Dubner & N. J. A. Sloane, Correspondence, 1991
R. K. Guy & N. J. A. Sloane, Correspondence, 1985
N. Kuosa, Source for 6380.
Des MacHale and Joseph Manning, Maximal runs of strictly composite integers, The Mathematical Gazette, 99, pp 213219 (2015).
Romeo Mestrovic, Euclid's theorem on the infinitude of primes: a historical survey of its proofs (300 BC2012) and another new proof, arXiv preprint arXiv:1202.3670 [math.HO], 2012.  From N. J. A. Sloane, Jun 13 2012
Hisanori Mishima, Factors of N!+1
Rudolf Ondrejka, The Top Ten: a Catalogue of Primal Configurations
Titus Piezas III, 2004. Solving Solvable Sextics Using Polynomial Decomposition
Maxie D. Schmidt, New Congruences and Finite Difference Equations for Generalized Factorial Functions, arXiv:1701.04741 [math.CO], 2017.
Apoloniusz Tyszka, A conjecture which implies that there are infinitely many primes of the form n!+1, Preprint, 2017.
Apoloniusz Tyszka, A common approach to the problem of the infinitude of twin primes, primes of the form n! + 1, and primes of the form n!  1, 2018.
Apoloniusz Tyszka, On sets X subset of N for which we know an algorithm that computes a threshold number t(X) in N such that X is infinite if and only if X contains an element greater than t(X), 2019.
Apoloniusz Tyszka, On sets X, subset of N, whose finiteness implies that we know an algorithm which for every n, element of N, decides the inequality max (X) < n, (2019).
Eric Weisstein's World of Mathematics, Factorial Prime
Eric Weisstein's World of Mathematics, Integer Sequence Primes
Index entries for sequences related to factorial numbers


EXAMPLE

3! + 1 = 7 is prime, so 3 is in the sequence.


MATHEMATICA

v = {0, 1, 2}; Do[If[ !PrimeQ[n + 1] && PrimeQ[n! + 1], v = Append[v, n]; Print[v]], {n, 3, 29651}]
Select[Range[100], PrimeQ[#! + 1] &] (* Alonso del Arte, Jul 24 2014 *)


PROG

(PARI) for(n=0, 1e4, if(ispseudoprime(n!+1), print1(n", "))) \\ Charles R Greathouse IV, Jun 16 2011
(MAGMA) [n: n in [0..800]  IsPrime(Factorial(n)+1)]; // Vincenzo Librandi, Oct 31 2018
(Python)
from sympy import factorial, isprime
for n in range(0, 800):
if isprime(factorial(n)+1):
print(n, end=', ') # Stefano Spezia, Jan 10 2019


CROSSREFS

Cf. A002982 (n!1 is prime), A064295. A088332 gives the primes.
Equals A090660  1.
Cf. A093804.
Sequence in context: A284046 A048412 A259428 * A294637 A295613 A232212
Adjacent sequences: A002978 A002979 A002980 * A002982 A002983 A002984


KEYWORD

hard,more,nonn,nice


AUTHOR

N. J. A. Sloane


EXTENSIONS

Term 6380 sent in by Jud McCranie, May 08 2000
Term 26951 from Ken Davis (kraden(AT)ozemail.com.au), May 24 2002
Term 110059 found by PrimeGrid around Jun 11 2011, submitted by Eric W. Weisstein, Jun 13 2011
Term 150209 by Rene Dohmen, Jun 09 2012


STATUS

approved



