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A082672 Numbers n such that (n! + 2)/2 is a prime. 56
2, 4, 5, 7, 8, 13, 16, 30, 43, 49, 91, 119, 213, 1380, 1637, 2258, 4647, 9701, 12258 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..19.

MATHEMATICA

Select[Range[10^2], PrimeQ[(#!+2)/2] &] (* Vladimir Joseph Stephan Orlovsky, Apr 29 2008 *)

PROG

(PARI) \\ x such that (x!+2)/2 is prime

xfactpk(n, k=2) = { for(x=2, n, y = (x!+k)/k; if(isprime(y), print1(x, ", ")) ) }

(MAGMA) [ n: n in [1..300] | IsPrime((Factorial(n)+2) div 2) ];

CROSSREFS

Cf. A089130.

Cf. n!/m-1 is a prime: A002982, A082671, A139056, A139199-A139205; n!/m+1 is a prime: A002981, A082672, A089085, A139061, A139058, A139063, A139065, A151913, A137390, A139071 (1<=m<=10).

Sequence in context: A005125 A232832 A272195 * A241808 A247865 A271505

Adjacent sequences:  A082669 A082670 A082671 * A082673 A082674 A082675

KEYWORD

easy,nonn

AUTHOR

Cino Hilliard, May 18 2003

EXTENSIONS

More terms from Don Reble, Dec 08 2003

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008

STATUS

approved

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Last modified August 21 16:04 EDT 2017. Contains 290890 sequences.