A071406 a(n) is the smallest multiplier of n! such that -1+a(n)*n! and 1+a(n)*n! are both primes.

4, 2, 1, 3, 2, 17, 7, 6, 3, 14, 29, 30, 48, 27, 9, 24, 12, 97, 78, 47, 71, 80, 55, 13, 57, 20, 81, 259, 108, 163, 81, 118, 63, 215, 173, 513, 420, 561, 537, 1162, 158, 33, 122, 286, 459, 391, 305, 288, 114, 307, 15, 680, 355, 365, 338, 70, 23

Offset

1,1

Links

Pierre CAMI, Table of n, a(n) for n = 1..300

Example

n=7: a(7)=7, 7!=5040, 7.7!=35280 and {35279,35281} are primes.

Mathematica

Table[fl=1; Do[s=(j!)*k; If[PrimeQ[s-1]&&PrimeQ[s+1]&&Equal[fl, 1], Print[{j, k}]; fl=0], {k, 1, 2*j^2}], {j, 0, 100}]
smnf[n_]:=Module[{k=1, f=n!}, While[!PrimeQ[k*f+1]||!PrimeQ[k*f-1], k++]; k]; Array[smnf, 60] (* Harvey P. Dale, May 24 2016 *)

Crossrefs

Cf. A063983, A071256, A060256.

Sequence in context: A309975 A010124 A271310 * A337063 A010311 A346972

Adjacent sequences: A071403 A071404 A071405 * A071407 A071408 A071409

Keyword

nonn

Author

Labos Elemer, May 24 2002

Status

approved