Question 1064462
 {... , -3, -1, 1, 3, ...}
<pre>
That's the set of all odd integers.

Since an odd integer times an odd integer is an odd integer,

The set is closed under multiplication (but not addition, subtraction,
or division)

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Of course there are some other contrived operations for which it is
closed, such as the "heart operation" &#9829;, "raising to the 
absolute value of a power"  defined by<font size=5><b> 

x &#9829; y = x<sup>|y|</sup></b></font>   

Edwin</pre>