@inproceedings{rainer1992helmholtz,
title={Helmholtz operators on harmonic manifolds},
author={Rainer Schimming, and Henrik Schlichtkrull},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203323339619506},
booktitle={Acta Mathematica},
volume={173},
number={2},
pages={235-258},
year={1992},
}
Rainer Schimming, and Henrik Schlichtkrull. Helmholtz operators on harmonic manifolds. 1992. Vol. 173. In Acta Mathematica. pp.235-258. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203323339619506.