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A071406 a(n) is the smallest multiplier of n! such that -1+a(n)*n! and 1+a(n)*n! are both primes. 3
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 (list; graph; refs; listen; history; text; internal format)
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 * A010311 A326485 A023528

Adjacent sequences:  A071403 A071404 A071405 * A071407 A071408 A071409

KEYWORD

nonn

AUTHOR

Labos Elemer, May 24 2002

STATUS

approved

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Last modified January 17 18:14 EST 2020. Contains 330987 sequences. (Running on oeis4.)