@inproceedings{pavel2005convergence,
title={Convergence of spectra of graph-like thin manifolds},
author={Pavel Exner, and Olaf Post},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020122240352050557},
booktitle={Journal of Geometry and Physics},
volume={54},
number={1},
pages={77-115},
year={2005},
}

Pavel Exner, and Olaf Post. Convergence of spectra of graph-like thin manifolds. 2005. Vol. 54. In Journal of Geometry and Physics. pp.77-115. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20191020122240352050557.