According to scholars such as V. Minorsky and others, the name Afghan appears in the 982 CE Hudud-al-Alam geography book. Al-Biruni referred to a group of Afghans in the 11th century as various tribes living on the western frontier mountains of Ancient India and Persia, which would be the area between the Hindu Kush mountains in Afghanistan and the Indus River in what is now Pakistan. According to other sources, some Pashtuns may be the Lost tribes of Israel who converted to Islam during the Arab Empire. Since the 13th century, some Pashtun tribes conquered areas outside their traditional Pashtun homeland by pushing deeper into South Asia, often forming kingdoms such as the Delhi Sultanate.