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