OFFSET
1,2
COMMENTS
Obviously, a(n) cannot be a prime number. 77 is the first term which is semiprime.
Numbers of the form 9*t are terms of this sequence for t > 1.
EXAMPLE
18 is a term because 18!!! = 524880 and 524880 mod 18^3 = 0.
27 is a term because 27!!! = 7142567040 and 7142567040 mod 27^3 = 0.
PROG
(PARI) tf(n) = prod(i=0, (n-1)\3, n-3*i);
k(n) = tf(n) % n^3;
for(n=1, 1e3, if(k(n)==0, print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Altug Alkan, Dec 15 2015
STATUS
approved