Let the function $f ( x ) = \frac { x ^ { 2 } } { 2 } - k \ln x$, $k > 0$\n(I) Find the monotonic intervals and extreme values of $f ( x )$;\n(II) Prove that if $f ( x )$ has a zero point, then $f ( x )$ has exactly one zero point on the interval $( 1 , \sqrt { e } )$.