is a subset of such that the product of any three distinct elements of is not a square. Determine the maximum number of elements in
%V0
$M$ is a subset of $\{1, 2, 3, \ldots, 15\}$ such that the product of any three distinct elements of $M$ is not a square. Determine the maximum number of elements in $M.$
Izvor: Međunarodna matematička olimpijada, shortlist 1994
Školjka 
is a subset of 
such that the product of any three distinct elements of 