@inproceedings{fritz1995picard,
title={Picard potentials and Hill's equation on a torus},
author={Fritz Gesztesy, and Rudi Weikard},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203326244909528},
booktitle={Acta Mathematica},
volume={176},
number={1},
pages={73-107},
year={1995},
}
Fritz Gesztesy, and Rudi Weikard. Picard potentials and Hill's equation on a torus. 1995. Vol. 176. In Acta Mathematica. pp.73-107. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203326244909528.