1. The Classical Groups;Weyl,1939
2. Über Biegungsinvarianten;Zorawski;Acta Math.,1892
3. On the invariants of quadratic differential forms;Haskins;Trans. Amer. Math. Soc.,1902
4. The Differential Invariants of Generalized Spaces;Thomas,1934
5. Conformal invariants;Fefferman,1985