Determine the least possible value of
![f(1998),](/media/m/1/f/2/1f2dbb246c8d901f0da973008f1c7b03.png)
where
![f](/media/m/9/9/8/99891073047c7d6941fc8c6a39a75cf2.png)
is a function from the set
![{\bf N}](/media/m/1/0/d/10dffb40270f30ffa4bf6b83a33a4348.png)
of positive integers into itself such that for all
![m,n\in {\bf N}](/media/m/7/0/d/70d1bd48efc21575177c71403e7a01fe.png)
,
%V0
Determine the least possible value of $f(1998),$ where $f$ is a function from the set ${\bf N}$ of positive integers into itself such that for all $m,n\in {\bf N}$,
$$f\left( n^{2}f(m)\right) =m\left[ f(n)\right] ^{2}.$$