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A153343
Numbers k such that 5*k + 4 is not prime.
7
0, 1, 2, 4, 6, 7, 8, 9, 10, 12, 13, 14, 16, 18, 19, 20, 22, 23, 24, 25, 26, 28, 30, 31, 32, 33, 34, 36, 37, 38, 40, 41, 42, 43, 44, 46, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 70, 72, 73, 74, 76, 78, 79, 80, 82, 84, 85, 86, 88
OFFSET
1,3
COMMENTS
Apart from a(0) = 0 and a(1) = 1 this sequence comprises those numbers k such that (5*k)!/(5*k + 4) is an integer. - Peter Bala, Jan 25 2017
LINKS
EXAMPLE
Distribution of the odd terms in the following triangular array:
1;
*, *;
*, *, 9;
*, *, *, *;
*, *, *, 19, *;
7, *, *, *, *, 33;
*, *, *, *, *, *, *;
*, *, 23, *, *, *, *, 57;
*, *, *, *, 41, *, *, *, *;
*, *, *, 37, *, *, *, *, 79, *;
13, *, *, *, *, 59, *, *, *, *, 105;
etc., where * marks the noninteger values of (4*h*k + 2*k + 2*h - 3)/5 with h >= k >= 1. - Vincenzo Librandi, Jan 17 2013
MAPLE
# produces the sequence apart from the initial terms 0 and 1
for n from 0 to 100 do
if irem(factorial(5*n), 5*n+4) = 0 then print(n); end if;
end do: # Peter Bala, Jan 25 2017
MATHEMATICA
Select[Range[0, 200], !PrimeQ[5*# + 4]&] (* Vincenzo Librandi, Jan 12 2013 *)
PROG
(Magma) [n: n in [0..150] | not IsPrime(5*n + 4)]; // Vincenzo Librandi, Jan 12 2013
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, Dec 24 2008
EXTENSIONS
Erroneous comment deleted by N. J. A. Sloane, Jun 23 2010
0 added by Arkadiusz Wesolowski, Aug 03 2011
STATUS
approved