A181766 - OEIS

%I #21 Sep 08 2022 08:45:54

%S 1,3,5,6,7,9,11,13,14,15,16,17,19,21,23,25,26,27,28,29,31,33,35,36,37,

%T 38,39,41,42,43,45,46,47,49,51,53,54,55,56,57,59,60,61,63,65,66,67,69,

%U 70,71,73,75,76,77,79,80,81,82,83,84,85,86,87,89,91,93,94,95,96,97,98

%N Numbers k such that 3*k + 7 is not prime.

%C One less than the associated value in A153282, two less than A153309. - _R. J. Mathar_, Jan 05 2011

%H Vincenzo Librandi, <a href="/A181766/b181766.txt">Table of n, a(n) for n = 1..1000</a>

%e Distribution of the even terms in the following triangular array:

%e *;

%e *, 6;

%e *, *,14;

%e *, *,*,*;

%e *,16,*,*,38;

%e *,*,28,*,*,54;

%e *,*, *,*,*, *,*;

%e *,26,*,*,60,*,*,94;

%e *,*,42,*,*,80,*,*,118;

%e *,*,*, *,*,*, *,*, *, *;

%e *,36,*,*,82,*,*,128,*,*,174;

%e *,*,56,*,*,106,*,*,156,*,*,206; etc.

%e where * marks the non-integer values of (4*h*k + 2*k + 2*h - 6)/3 with h >= k >= 1. - _Vincenzo Librandi_, Jan 15 2013

%t Select[Range[0, 100], !PrimeQ[3 # + 7] &] (* _Vincenzo Librandi_, Oct 15 2012 *)

%o (Magma) [n: n in [0..100] | not IsPrime(3*n + 7)]; // _Vincenzo Librandi_, Oct 15 2012

%Y Cf. A089953 (3*n+7 is a prime).

%Y Cf. A153282, A153309.

%K nonn,easy

%O 1,2

%A _Vincenzo Librandi_