%I #23 Sep 08 2022 08:45:39

%S 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,

%T 34,35,37,38,39,41,42,43,44,45,47,49,50,51,52,53,55,56,57,58,59,60,61,

%U 62,63,64,65,66,67,68,69,71,73,74,75,77,79,80,81,83,85,86,87,89,91,92,93

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

%H Vincenzo Librandi, <a href="/A153088/b153088.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = A153343(n) + 1. - _Peter Bala_, Jan 25 2017

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

%e 2;

%e *, *;

%e *, *, 10;

%e *, *, *, *;

%e *, *, *, 20, *;

%e 8, *, *, *, *, 34;

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

%e *, *, 24, *, *, *, *, 58;

%e *, *, *, *, 42, *, *, *, *;

%e *, *, *, 38, *, *, *, *, 80, *;

%e 14, *, *, *, *, 60, *, *, *, *, 106;

%e 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

%p # produces the sequence apart from the initial terms 1 and 2

%p for n from 0 to 100 do

%p if irem(factorial(5*n), 5*n+4) = 0 then print(n+1); end if;

%p end do: # _Peter Bala_, Jan 25 2017

%t Select[Range[1, 200], !PrimeQ[5 # - 1] &] (* _Vincenzo Librandi_, Oct 11 2012 *)

%o (Magma) [n: n in [1..100] | not IsPrime(5*n-1)]; // _Vincenzo Librandi_, Oct 11 2012

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

%K nonn,easy

%O 1,2

%A _Vincenzo Librandi_, Jan 03 2009

%E First 29 replaced with 20, 4 replaced with 44, extended by _R. J. Mathar_, Jan 05 2009

%E Erroneous comment deleted by _N. J. A. Sloane_, Jun 23 2010