Finding Not-Quite-Prime Numbers
J, 17

*/&lt;:$&amp;q:(&lt;:,],&gt;:)


Returns booleans encoded as process return codes: zero for true, nonzero for false.  Sample use:

   */&lt;:$&amp;q:(&lt;:,],&gt;:) 7
0
   */&lt;:$&amp;q:(&lt;:,],&gt;:) 8
0
   */&lt;:$&amp;q:(&lt;:,],&gt;:) 15
3