*The fourth in a series. See the November 20 entry, “What is algebra?” for the first, the December 13 entry “When did algebra begin?” for the second, and the December 19 entry “The golden age of Arabic mathematics” for the third.*

Abū ʿAbdallāh Muḥammad ibn Mūsā al-Khwārizmī (c.780 – c.850 CE) was one of the most significant figures in the development of modern algebra. Yet we know virtually nothing about his life.

There is even some confusion in the literature as to his full name. Most present-day sources give it as *Abū **ʿ**Abdallāh Mu**ḥ**ammad ibn Mūsā al-Khwārizmī*, which can be translated as “Father of ʿAbdallāh, Mohammed, son of Moses, native of the town of al-Khwārizmī”. References to *Abū Ja**ʿ**far Mu**ḥ**ammad ibn Mūsā al-Khwārizmī* are erroneous in this context; that was a different person

Al-Khwārizmī wrote several books, two of which had a huge impact on the growth of mathematics, one focused on arithmetic, the other on algebra. He aimed both at a much wider audience than just his fellow scholars. As with Euclid and his *Elements*, it is not clear whether al-Khwārizmī himself developed some of the methods he desribed in his books, in addition to gathering together the work of others, though a later author, Abū Kāmil, suggested that his famous predecessor did develop some of the methods he presented in his books.

The first of al-Khwārizmī’s two most significant books, written around 825, described Hindu-Arabic arithmetic. Its original title is not known, and it may not have had one. No original Arabic manuscripts exist, and the work survives only through a Latin translation, which was most likely made in the 12^{th} century by Adelard of Bath. The original Latin translation did not have a title either, but the Italian bibliophile Baldassare Boncompagni gave it one when he published a printed edition in the 19^{th} century: *Algoritmi de numero Indorum *(“al-Khwārizmī on the Hindu Art of Reckoning”). The Latinized version of al-Khwārizmī’s name in this title (*Algoritmi*) gave rise to our modern word “algorithm” for a set of rules specifying a calculation. In English, the work is sometimes referenced as *On the Calculation with Hindu Numerals,* but it is most commonly referred to simply as “al-Khwārizmī’s *Arithmetic*.”

Al-Khwārizmī’s second pivotal book, completed around 830, was *al-Kitab al-mukhtasar fi hisab al-jabr wa’l-muq**ā**bala*. The phrase *al-jabr wa’l-muqābalah* translates literally as “restoration and confrontation,” or more loosely as “reducing (or solving) an equation.” The title of the book translates literally as “The Abridged Book on Calculation by Restoration and Confrontation”, but a more colloquial rendering would thus be “The Abridged Book on Algebra”. It is an early treatise on what we now call “algebra,” that name coming from the term *al-jabr* in the title. Scholars today usually refer to this book simply as “Al-Khwārizmī’s *Algebra*.” There are seven Arabic manuscripts known, not all complete. One complete Arabic copy is kept at Oxford and a Latin translation is kept in Cambridge. Two copies are in Afghanistan.

In *Algebra*, al-Khwārizmī described (but did not himself develop) a systematic approach to solving linear and quadratic equations, providing a comprehensive account of solving polynomial equations up to the second degree.

The *Algebra* was translated into Latin by Robert of Chester in 1145, by Gherardo of Cremona around 1170, and by Guglielmo de Lunis around 1250. In 1831, Frederic Rosen published an English language translation. In his preface, Rosen wrote:

ABU ABDALLAH MOHAMMED BEN MUSA, of Khowarezm, who it appears, from his preface, wrote this Treatise at the command of the Caliph AL MAMUN, was for a long time considered as the original inventor of Algebra. … … … From the manner in which our author [al-Khwārizmī], in his preface, speaks of the task he had undertaken, we cannot infer that he claimed to be the inventor. He says that the Caliph AL MAMUN encouraged him to write a popular work on Algebra: an expression which would seem to imply that other treatises were then already extant.

In fact, algebra (as al-Khwārizmī described it in his book) was being transmitted orally and being used by people in their jobs before he or anyone else started to write it down. Several authors wrote books on algebra during the ninth century besides al-Khwārizmī, all having the virtually identical title *Kitāb al-ğabr wa-l-muqābala*. Among them were Abū Hanīfa al-Dīnawarī, Abū Kāmil Shujā ibn Aslam, Abū Muḥammad al-ʿAdlī, Abū Yūsuf al-Miṣṣīṣī, ‘Abd al-Hamīd ibn Turk, Sind ibn ʿAlī, Sahl ibn Bišr, and Šarafaddīn al-Tūsī.

In addition to his two books on mathematics, al-Khwārizmī wrote a revised and completed version of Ptolemy’s *Geography*, consisting of a general introduction followed by a list of 2,402 coordinates of cities and other geographical features. Titled *Kitāb **ṣ**ūrat al-Ar**ḍ** *(“Book on the appearance of the Earth” or “The image of the Earth”), he finished it in 833. There is only one surviving Arabic copy, which is kept at the Strasbourg University Library. A Latin translation is kept at the Biblioteca Nacional de España in Madrid.

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COMING UP NEXT: Al-Khwārizmī’s answer to that perennial student question, “What is algebra good for?” Plus a look at the contents of his seminal book, including an explanation of what exactly was being “restored” in the process for which al-Khwārizmī’s Arabic term was *al-jabr*.

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**Al-Khwārizmī on National Public Radio:** I talked about al-Khwārizmī and the birth of algebra with host Scott Simon in my occasional “Math Guy” slot on NPR’s Weekend Edition on December 24.