@inproceedings{einar1931on,
title={On the characteristic values of linear integral equations},
author={Einar Hille, and J. D. Tamarkin},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203057460769474},
booktitle={Acta Mathematica},
volume={57},
number={1},
pages={1-76},
year={1931},
}
Einar Hille, and J. D. Tamarkin. On the characteristic values of linear integral equations. 1931. Vol. 57. In Acta Mathematica. pp.1-76. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203057460769474.