A240622 Least number k such that k!/n - 1 is prime.

3, 3, 4, 4, 5, 4, 7, 4, 6, 5, 15, 6, 13, 7, 5, 9, 38, 8, 21, 5, 7, 19, 27, 6, 15, 14, 10, 7, 30, 5, 31, 8, 12, 18, 8, 6, 47, 53, 13, 5, 127, 10, 67, 11, 16, 27, 51, 8, 14, 26, 17, 16, 77, 9, 23, 7, 184, 56, 123, 6, 66, 203, 7, 9, 13, 13, 74, 42, 26, 7, 75, 9, 205

Offset: 1

Author

Derek Orr, Apr 09 2014

Keywords

• nonn

Comments

a(263) > 5000. - Jinyuan Wang, Mar 31 2020

Examples

			1!/1 - 1 = 0 is not prime. 2!/1 - 1 = 1 is not prime. 3!/1 - 1 = 5 is prime. Thus, a(1) = 3.
		

Links

• Jinyuan Wang, Table of n, a(n) for n = 1..262

Crossrefs

Cf. A139072, A139186.

Programs

Mathematica

lnk[n_]:=Module[{k=1},While[!PrimeQ[k!/n-1],k++];k]; Array[lnk,80] (* Harvey P. Dale, Aug 31 2015 *)

PARI

a(n) = {for(k=1, oo, s=k!/n-1; if(floor(s)==s, if(ispseudoprime(s), return(k)))); }