A153355 -id:A153355 - cp's OEIS frontend

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

Vincenzo Librandi, Jan 03 2009

			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

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

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

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

# 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

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

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

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

A153877 Numbers n such that 5n-1 and 5n+1 are both prime.

Original entry on oeis.org

6, 12, 30, 36, 48, 54, 84, 114, 120, 132, 162, 204, 210, 246, 258, 264, 324, 390, 426, 462, 468, 510, 546, 558, 594, 600, 624, 660, 666, 672, 678, 708, 786, 804, 810, 846, 852, 930, 960, 1002, 1020, 1056, 1104, 1128, 1170, 1176, 1218, 1254, 1260, 1272

Offset: 1

Vincenzo Librandi, Jan 03 2009

a(n) is an integer multiple of 6. - Alois P. Heinz, Jan 04 2009

			n=6, 5n-1=29, 5n+1=31; n=30, 5n-1=149, 5n+1=151.

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

Cf. A153355, A153088.

[ n: n in [0..2000] | IsPrime(5*n-1) and IsPrime(5*n+1)]; // Vincenzo Librandi, Jul 31 2012

a:= proc(n::posint) option remember; local m; if n=1 then 6 else for m from a(n-1)+6 by 6 while not (isprime(5*m-1) and isprime(5*m+1)) do od; m fi end: seq(a(n), n=1..100); # Alois P. Heinz, Jan 04 2009

Select[Range[0,100001],PrimeQ[5#-1]&& PrimeQ[5#+1]&] (* Vincenzo Librandi, Jul 31 2012 *)

Corrected and extended by Alois P. Heinz and R. J. Mathar, Jan 04 2009

cp's OEIS Frontend

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

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