@inproceedings{victor1966the$d$-step,
title={The$d$-step conjecture for polyhedra of dimension$d$<6},
author={Victor Klee, and David W. Walkup},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203209983887021},
booktitle={Acta Mathematica},
volume={117},
number={1},
pages={53-78},
year={1966},
}

Victor Klee, and David W. Walkup. The$d$-step conjecture for polyhedra of dimension$d$<6. 1966. Vol. 117. In Acta Mathematica. pp.53-78. http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20170108203209983887021.