@inproceedings{george1933analytic,
title={Analytic theory of singular difference equations},
author={George D. Birkhoff, and W. J. Trjitzinsky},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203101456672507},
booktitle={Acta Mathematica},
volume={60},
number={1},
pages={1-89},
year={1933},
}
George D. Birkhoff, and W. J. Trjitzinsky. Analytic theory of singular difference equations. 1933. Vol. 60. In Acta Mathematica. pp.1-89. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203101456672507.