Shoe Surface

$\displaystyle x(u,v)$	$\textstyle =$	$\displaystyle u$
$\displaystyle y(u,v)$	$\textstyle =$	$\displaystyle v$
$\displaystyle z(u,v)$	$\textstyle =$	$\displaystyle {\textstyle{1\over 3}}u^3-{\textstyle{1\over 2}}v^2.$