Al-Khwarizmi (c.780-c.850), Persian scholar. One of the major advances made by Muhammad ibn Musa al-Khwarizmi was in algebra where he proposed the use of a symbol (an empty circle or 'sifr') to represent the absence of a number. This is known as zero and was an important advance in the simplification and representation of calculations. Al-Khwarizmi worked in Baghdad, and also produced works in astronomy and geography.