《后汉书·列传·章帝八王列传》
河间孝王刘开，是永元二年封的，分乐成、勃海、涿郡为国。
河间孝王开，以永元二年封，分乐成、勃海、涿郡为国。
Liu Kai, the filial king of Hejian, was granted a title in the second year of Yongyuan, and the states of Lecheng, Bohai, and Zhuojun were divided into his country.