A153088 Numbers k such that 5*k - 1 is not prime.

1, 2, 3, 5, 7, 8, 9, 10, 11, 13, 14, 15, 17, 19, 20, 21, 23, 24, 25, 26, 27, 29, 31, 32, 33, 34, 35, 37, 38, 39, 41, 42, 43, 44, 45, 47, 49, 50, 51, 52, 53, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 71, 73, 74, 75, 77, 79, 80, 81, 83, 85, 86, 87, 89, 91, 92, 93

Offset: 1

Author

Vincenzo Librandi, Jan 03 2009

Keywords

• nonn
• easy

Examples

			Distribution of the even terms in the following triangular array:
   2;
   *,   *;
   *,   *,  10;
   *,   *,   *,   *;
   *,   *,   *,  20,   *;
   8,   *,   *,   *,   *,  34;
   *,   *,   *,   *,   *,   *,   *;
   *,   *,  24,   *,   *,   *,   *,  58;
   *,   *,   *,   *,  42,   *,   *,   *,   *;
   *,   *,   *,  38,   *,   *,   *,   *,  80,   *;
  14,   *,   *,   *,   *,  60,   *,   *,   *,   *, 106;
etc., where * marks the noninteger values of (4*h*k + 2*k + 2*h + 2)/5 with h >= k >= 1. - _Vincenzo Librandi_, Jan 15 2013
		

Links

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

Crossrefs

Cf. A153355, A014076, A153170, A045751, A095277, A153329, A153343.

Programs

Magma

[n: n in [1..100] | not IsPrime(5*n-1)]; // Vincenzo Librandi, Oct 11 2012

Maple

# produces the sequence apart from the initial terms 1 and 2
for n from 0 to 100 do
  if irem(factorial(5*n), 5*n+4) = 0 then print(n+1); end if;
end do: # Peter Bala, Jan 25 2017

Mathematica

Select[Range[1, 200], !PrimeQ[5 # - 1] &] (* Vincenzo Librandi, Oct 11 2012 *)

Formula

a(n) = A153343(n) + 1. - Peter Bala, Jan 25 2017

Extensions

First 29 replaced with 20, 4 replaced with 44, extended by R. J. Mathar, Jan 05 2009

Erroneous comment deleted by N. J. A. Sloane, Jun 23 2010