Ulrich Libbrecht concluded: "It is obvious that this procedure is a Chinese invention ... the method was not known in India". He said, Fibonacci probably learned of it from Arabs, who perhaps borrowed from the Chinese. The extraction of square and cube roots along similar lines is already discussed by Liu Hui in connection with Problems IV.16 and 22 in "Jiu Zhang Suan Shu", while Wang Xiaotong in the 7th century supposes his readers can solve cubics by an approximation method described in his book Jigu Suanjing.