1. Clifford W.K.: Applications of Grassmann’s extensive algebra. Amer. J. Math. 1, 350–358 (1878)
2. W. R. Hamilton, Lectures on Quaternions. Hodges and Smith, Dublin, 1853.
3. H. Grassmann, Die Ausdehnungslehre. Enslin, Berlin, 1862.
4. W. K. Clifford, On the classification of geometric algebras. In: R. Tucker, ed., Mathematical Papers by William Kingdon Clifford, Macmillan, London, 1982, pp. 397–401.
5. R. Lipschitz, Untersuchungen über die Summen von Quadraten. Max Cohen und Sohn, Bonn, 1886.