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

1,2

Links

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

Formula

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

Example

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

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 *)

Prog

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

Crossrefs

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

Sequence in context: A346127 A039103 A228082 * A162535 A279814 A047490

Adjacent sequences: A153085 A153086 A153087 * A153089 A153090 A153091

Keyword

nonn,easy

Author

Vincenzo Librandi, Jan 03 2009

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

Status

approved