Oleinik O A <br/><br/>
Second order equations with nonnegative characteristic form. 

 -  1973 
<br/><br/><label>Standard No.: </label>17162 
<br/><br/><label>Source: </label> Textual 
<br/><br/><label>Dewey Class. No.: </label>B334,1,2, L3