Levy, Azriel. <br/><br/>
A hierarchy of formulas in set theory/ 
by Azriel Lévy. 
 - Providence, R.I.:  American Mathematical Society,  1965. 
 - 76 p.:  26 cm. 
 - Memoirs of the American Mathematical Society;  no. 57 .
<br/><br/> Cover title. 
<br/><br/>Bibliography: p. 74-76. 
<br/><br/><label>Subjects--Topical Terms: </label><br/>Set theory.