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

1,2

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.

Links

Table of n, a(n) for n=1..72.

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

Crossrefs

Cf. A034693.

Sequence in context: A232091 A364066 A164798 * A281072 A226951 A251557

Adjacent sequences: A087551 A087552 A087553 * A087555 A087556 A087557

Keyword

nonn

Author

Amarnath Murthy, Sep 13 2003

Extensions

Edited by Ray Chandler and Don Reble, Sep 16 2003

Status

approved