MathSciDoc: An Archive for Mathematician ∫

@inproceedings{george1930formal,
  title={Formal theory of irregular linear difference equations},
  author={George D. Birkhoff},
  url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203054131029450},
  booktitle={Acta Mathematica},
  volume={54},
  number={1},
  pages={205-246},
  year={1930},
}