"Probably the most striking testimony from Arabic sources is that given by the Arabic traveler and scholar Mohammed ibn Ahmed, Abū ’l-Rīahān al-Bīrūnī (973-1048), who spent many years in Hindustan. He wrote... the “Book of the Ciphers,” unfortunately lost, which treated... of the Hindu art of calculating... being versed in Arabic, Persian, Sanskrit, Hebrew, and Syriac, as well as in astronomy, chronology, and mathematics. In his work on India he... states explicitly that the Hindus of his time did not use the letters of their alphabet for numerical notation, as the Arabs did. He also states that the numeral signs called aṅka had different shapes in various parts of India, as was the case with the letters. In his Chronology of Ancient Nations he gives the sum of a geometric progression... in three different systems..."