A173750 a(n) = smallest positive m such that k*m+1 is prime for k=2..n.

1, 1, 2, 330, 714, 13530, 192660, 512820, 4601310, 863815050, 262428279750, 2169289182060, 2169289182060, 2169289182060, 4646092391146085880

Offset: 1

Author

Zak Seidov, Nov 26 2010

Keywords

• nonn
• more

Crossrefs

Cf. A088250 (smallest number k such that r*k+1 is prime for all r = 1 to n).

Programs

Magma

S:=[]; m:=1; for n in [1..11] do while not forall(t){ k: k in [2..n] | IsPrime(k*m+1) } do m+:=1; end while; Append(~S, m); end for; S; // Klaus Brockhaus, Nov 26 2010

Mathematica

a[n_] := Block[{k = If[n < 4, 1, 6], s}, s = k; While[! AllTrue[k Range[2, n] + 1, PrimeQ], k += s]; k]; Array[a, 9] (* Giovanni Resta, Mar 30 2017 *)

Extensions

a(12)-a(15) from Giovanni Resta, Mar 31 2017