Smith, David Eugene; Karpinski, Louis Charles (1911), The Hindu–Arabic Numerals, Boston and London: Ginn and Company, p. 128, archived from the original on 2023-03-13, retrieved 2016-03-02.
Smith, David Eugene; Karpinski, Louis Charles (1911), The Hindu–Arabic Numerals, Boston and London: Ginn and Company, p. 128, archived from the original on 2023-03-13, retrieved 2016-03-02.