A082671
Numbers n such that (n!-2)/2 is a prime.
Original entry on oeis.org
3, 4, 5, 6, 9, 31, 41, 373, 589, 812, 989, 1115, 1488, 1864, 1918, 4412, 4686, 5821, 13830
Offset: 1
(4!-2)/2 = 11 is a prime.
Cf. n!/m-1 is a prime:
A002982,
A082671,
A139056,
A139199,
A139200,
A139201,
A139202,
A139203,
A139204,
A139205; n!/m+1 is a prime:
A002981,
A082672,
A089085,
A139061,
A139058,
A139063,
A139065,
A151913,
A137390,
A139071
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[n: n in [1..600]| IsPrime((Factorial(n)-2) div 2)]; // Vincenzo Librandi, Feb 18 2015
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Select[Range[0, 14000], PrimeQ[(#! - 2) / 2] &] (* Vincenzo Librandi, Feb 18 2015 *)
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xfactpk(n,k=2) = { for(x=2,n, y = (x!-k)/k; if(isprime(y),print1(x", ")) ) }
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008
A139199
Numbers k such that (k!-4)/4 is prime.
Original entry on oeis.org
4, 5, 6, 7, 8, 10, 15, 18, 23, 157, 165, 183, 184, 362, 611, 908, 2940, 6875, 9446, 16041
Offset: 1
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).
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a = {}; Do[If[PrimeQ[(n! - 4)/4], Print[a]; AppendTo[a, n]], {n, 1, 184}]; a (*Artur Jasinski*)
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is(n)=n>3 && isprime(n!/4-1) \\ Charles R Greathouse IV, Apr 29 2015
A290121
Primes of the form k!/8 - 1.
Original entry on oeis.org
2, 89, 5039, 453599, 4989599, 2615348735999, 15205637551103999, 32327905188896022580370544394201497496149704048639999999999, 68043117384680383000466155030980344080536633048599859316607515858708885143551999999999999999
Offset: 1
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Select[Table[k! / 8 - 1, {k, 4, 100}], PrimeQ[#]&]
Showing 1-3 of 3 results.
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