Alexandroff, Paul. <br/><br/>
Elementary Concepts of Topology /  
translated by Alan E. Farley. 
 - 1st ed /  
 - New York 10014 :   Dover Publications Inc, ,   1982 
 - p.57 :  
<br/><br/><label>ISBN: </label>0-486-60747-X 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Topology
<br/><br/><label>Dewey Class. No.: </label>B316  / K1A