@inproceedings{h.1955convergent,
title={Convergent solutions of ordinary linear homogeneous differential equations in the neighborhood of an irregular singular point},
author={H. L. Turrittin},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203142038999811},
booktitle={Acta Mathematica},
volume={93},
number={1},
pages={27-66},
year={1955},
}

H. L. Turrittin. Convergent solutions of ordinary linear homogeneous differential equations in the neighborhood of an irregular singular point. 1955. Vol. 93. In Acta Mathematica. pp.27-66. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203142038999811.