@inproceedings{james1959on,
title={On a fundamental theorem of the calculus of variations},
author={James Serrin},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203152264220886},
booktitle={Acta Mathematica},
volume={102},
number={1},
pages={1-22},
year={1959},
}