A153040 Numbers n>3 such that 2*n-5 is not a prime.

7, 10, 13, 15, 16, 19, 20, 22, 25, 27, 28, 30, 31, 34, 35, 37, 40, 41, 43, 45, 46, 48, 49, 50, 52, 55, 58, 60, 61, 62, 63, 64, 65, 67, 69, 70, 73, 74, 75, 76, 79, 80, 82, 83, 85, 87, 88, 90, 91, 94, 95, 96, 97, 100, 103, 104, 105, 106, 107, 109, 110, 111

Offset: 1

Author

Vincenzo Librandi, Dec 17 2008

Keywords

• nonn
• easy

Comments

One less than the associated number in A153039; one more than that in A153043. - R. J. Mathar Dec 20 2008

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A089253, A153039.

Programs

Magma

[n: n in [4..120]|not IsPrime(2*n-5)]; // Vincenzo Librandi, Aug 30 2012

Mathematica

Select[Range[4,200],!PrimeQ[2*#-5]&] (* Vladimir Joseph Stephan Orlovsky, Feb 09 2012 *)

Formula

Let p = prime number n = (p^2+5)/2 mod (p)

Extensions

Flipped sign in definition. - R. J. Mathar, Dec 20 2008