The history of mathematical development lacks some evidence. There are still debates about certain mathematical classics. For example, the "Zhoubi Suanjing" dates around 1200–1000 BC, yet many scholars believed it was written between 300 and 250 BC. The "Zhoubi Suanjing" contains an in-depth proof of the "Gougu Theorem" (a special case of the Pythagorean Theorem) but focuses more on astronomical calculations. However, the recent archaeological discovery of the Tsinghua Bamboo Slips, dated c. 305 BC, has revealed some aspects of pre-Qin mathematics, such as the first known decimal multiplication table.