@inproceedings{shujun2008the,
title={THE RESEARCH ON THE DIFFICULTY GRADE AND PIECING SKILLS OF TANGRAM},
author={Shujun Cai, Yizhi Jiang, and Minyu Zeng},
url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20160813215154756648063},
year={2008},
}