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A240133 Least number k such that n!/k + 1 and n!/k - 1 are twin primes. 1
1, 2, 2, 3, 12, 12, 48, 8, 5, 54, 44, 6, 24, 39, 6, 81, 20, 30, 19, 25, 380, 28, 264, 50, 52, 250, 35, 385, 20, 77, 182, 405, 77, 605, 143, 720, 144, 722, 96, 713, 46, 403, 98, 4508, 90, 77, 560, 806, 64, 665, 2376, 1785, 893, 1235, 4278, 159, 66, 1326, 1806, 429, 475 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,2

LINKS

Giovanni Resta, Table of n, a(n) for n = 3..350

EXAMPLE

6!/1-1 (719) and 6!/1+1 (721) are not both prime. 6!/2-1 (359) and 6!/2+1 (361) are not both prime. 6!/3-1 (239) and 6!/3+1 (241) are both prime. thus, a(6) = 3.

MATHEMATICA

lnk[n_]:=Module[{nf=n!, k=1}, While[!AllTrue[nf/k+{1, -1}, PrimeQ], k++]; k]; Array[lnk, 70, 3] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, May 18 2015 *)

PROG

(PARI) f(n)=for(k=1, n!, if(floor(n!/k-1)==n!/k-1 && floor(n!/k+1)==n!/k+1, if(ispseudoprime(n!/k-1) && ispseudoprime(n!/k+1), return(k))))

CROSSREFS

Cf. A212281, A212282.

Sequence in context: A134243 A182779 A199673 * A293445 A126339 A268725

Adjacent sequences:  A240130 A240131 A240132 * A240134 A240135 A240136

KEYWORD

nonn

AUTHOR

Derek Orr, Apr 02 2014

STATUS

approved

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Last modified September 15 02:19 EDT 2019. Contains 327062 sequences. (Running on oeis4.)