A171751 Maximum k with 1 <= k <= n such that nk + 1 is prime (or 0 if no such prime exists).

1, 2, 2, 4, 2, 6, 6, 5, 8, 10, 8, 9, 12, 14, 14, 16, 14, 17, 12, 20, 20, 21, 20, 24, 24, 26, 20, 27, 18, 27, 22, 29, 30, 33, 30, 36, 34, 32, 38, 40, 38, 41, 40, 33, 40, 43, 44, 45, 34, 47, 50, 49, 44, 54, 54, 56, 54, 57, 54, 59, 58, 59, 62, 54, 54, 66, 66, 65, 58, 69, 60, 71, 64

Offset

1,2

Comments

It is conjectured that nonzero terms exist for each n: see A089727, the main entry.

a(n) = n iff n is a term of A005574.

Links

Table of n, a(n) for n=1..73.

Mathematica

mk[n_]:=Module[{k=n}, While[!PrimeQ[n*k+1], k--]; If[k<1, 0, k]]; Array[mk, 80] (* Harvey P. Dale, Sep 24 2017 *)

Prog

(PARI) {A171751(n) = local(k); k=n; while(!isprime(n*k+1) && k>0, k--); k}

Crossrefs

Cf. A089727 (corresponding primes), A005574.

Sequence in context: A078429 A360523 A358039 * A124676 A076249 A062170

Adjacent sequences: A171748 A171749 A171750 * A171752 A171753 A171754

Keyword

nonn

Author

Rick L. Shepherd, Dec 17 2009

Status

approved