《水经注·卷六》
汾水又往西南流经长惰县老城南，汉高帝十一年前,把这里立为侯国，封给杜恬。
汾水又西南径长修故城南，汉高帝十一年以为侯国，封杜恬也。
Fen River continues southwestward and flows past the south of the old town of Changduo County, before eleven years into reign of Han Gaodi, it was established as a principality and enfeoffed to Du Tian.