回複：求教：小學5年級算數題目求證

This is equivalent to find the largest set such that no two integers can
multiple to a total square.

Here is a largest set of 17 numbers:

1,
2, 3, 4, 6,
5, 15,
7, 21,
11, 22,
13, 26,
17, 19, 23, 29

So the answer to the question is 18.