Weakly differentiable functions : 
Ziemer, William P. 
 - New York :  Springer-Verlag,  1989 
 - xvi, 308 p. ;  25 cm. 
 - Graduate texts in mathematics ;  120 .
<br/><br/> Includes index. 
<br/><br/><label>ISBN: </label>0387970177 (alk. paper) :