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2. P. L. Chebyshev, Theory of Congruences [in Russian], Obshchestvennaya Pol'za, Saint Petersburg (1879).
3. C. F. Gauss, Disquisitiones Arithmetique, Verlag Fischer, Leipzig (1801).
4. C. F. Gauss, Werke (1863), X(1), p. 571.
5. H. Poincaré, Oeuvres de Henri Poincaré, Tom 2, Gauthier-Villars, Paris (1916).