A038700 Smallest prime == -1 (mod n).

2, 3, 2, 3, 19, 5, 13, 7, 17, 19, 43, 11, 103, 13, 29, 31, 67, 17, 37, 19, 41, 43, 137, 23, 149, 103, 53, 83, 173, 29, 61, 31, 131, 67, 139, 71, 73, 37, 233, 79, 163, 41, 257, 43, 89, 137, 281, 47, 97, 149, 101, 103, 211, 53, 109, 167, 113, 173, 353, 59, 487, 61, 251

Offset: 1

Author

David W. Wilson

Keywords

• nonn
• easy

Links

• Vladimir Pletser, Table of n, a(n) for n = 1..20000 (first 1000 terms from Joerg Arndt)

Crossrefs

Cf. A034694.

Programs

Mathematica

f[n_] := Block[{k = n - 1}, While[ !PrimeQ@k, k += n]; k]; Array[f, 63] (* Robert G. Wilson v, Jun 09 2009 *)
With[{prs=Prime[Range[100]]},Table[SelectFirst[prs,Mod[#,n]==n-1&],{n,70}]] (* Harvey P. Dale, Apr 26 2023 *)

PARI

N=10^8;  default(primelimit,N);
a(n)=forprime(p=2,10^7, if(p%n==n-1,return(p)));
/* Joerg Arndt, Apr 16 2013 */

PARI

a(n)=my(j);while(!isprime(j++*n-1),);j*n-1 \\ Charles R Greathouse IV, Apr 18 2013