Differential equations and the calculus of variations 
Ėlʹsgolʹt︠s︡, L. Ė. 
 - Moscow,  Mir Publishers,  1973 
 - 440 p. with diagrs.  22 cm. 
<br/><br/> Translation of Different︠s︡ialʹnye uravnenii︠a︡ i variat︠s︡ionnoe ischislenie. 
<br/><br/><label>ISBN: </label>NULL