A087554 a(n) = smallest number k >= n such that nk + 1 is a prime.

1, 2, 4, 4, 6, 6, 10, 9, 12, 10, 18, 13, 24, 14, 16, 16, 18, 21, 22, 20, 22, 28, 26, 24, 28, 26, 28, 34, 32, 33, 36, 36, 34, 37, 42, 36, 40, 39, 48, 40, 42, 46, 46, 47, 48, 51, 50, 54, 52, 51, 56, 55, 56, 54, 58, 56, 58, 61, 60, 67, 66, 63, 66, 67, 68, 66, 70, 75, 70, 79, 72, 79

Offset: 1

Author

Amarnath Murthy, Sep 13 2003

Keywords

• nonn

Comments

Conjectures: (1) k < 2n. (2) For every r, there exists a number S, nr < S < n(r+1) such that nS + 1 is prime.

Crossrefs

Cf. A034693.

Programs

Maple

for n from 1 to 120 do k := n: while(not isprime(n*k+1)) do k := k+1:od:a[n] := k:od:seq(a[l],l=1..120); # Sascha Kurz

Extensions

Edited by Ray Chandler and Don Reble, Sep 16 2003