Idempotent operations (functions) are:f (maps X into Y, and if Y is a subset of X) then f is idempotent if, for all x in X; f(f(x)) = f(x).(see: <a href="http://en.wikipedia.org/wiki/Idempotent">http://en.wikipedia.org/wiki/Idempotent</a> )So, basicly, f is idempotent if f(f(x))=f(x), f(x)=x is idempotent, but f(x)=x*x is not!Artur