Why were Yang Hui’s mathematical works lost and later rediscovered?

Yang Hui’s mathematical works were lost during a period of decline in Chinese mathematics between the 14th and 16th centuries. As the use of counting rods—the basis for traditional algebraic place-value notations—faded in favor of the abacus, earlier sophisticated mathematical methods became difficult to understand and were eventually forgotten. By the 17th century, few ancient mathematical texts remained accessible to scholars.12

The rediscovery of Yang Hui’s treatises occurred during an 18th-century movement in which Chinese scholars, spurred by an interest in synthesizing traditional methods with Western algebra, actively sought out and reconstructed ancient texts. Ruan Yuan, a compiler of mathematical biographies, was instrumental in this effort. He first identified fragments of Yang’s Xiangjie jiuzhang suanfa within a handwritten copy of a Ming dynasty encyclopedia and later discovered a Song dynasty edition of Yang Hui suanfa in Suzhou. These philological efforts, often utilizing sources like the Yongle dadian, allowed for the restoration of these significant works.123