@inproceedings{per1972a,
title={A counterexample to the approximation problem in Banach spaces},
author={Per Enflo},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203228742034150},
booktitle={Acta Mathematica},
volume={130},
number={1},
pages={309-317},
year={1972},
}