1. e.g.: Smirnov, I.: Lehrgang der Höheren Mathematik, Vol. 5. Berlin: VEB Deutscher Verlag der Wissenschaften 1962.
2. Sufficient conditions were given by Fermat, P.: Varia opera mathematica; Tolosae, apud Ioannem Pech, Comitiorium Fuxensium typographum, juxta Collegium P.P. Societatis Jesu MDCLXXIX.;
3. Necessary and sufficient conditions by Cauchy, A.: Mém. Sc. Math. Phys. de l'Institut de France, (1),14, 1813–5; 177;
4. A formula for theN g was found by: Gauss, C. F.: Disq. Arith., Arith., Art. 291, (1801); Werke, I, 343 (1863);
5. see also Dickson, L. E.: History of the theory of numbers, Vol. II. Washington: Carnegie Institution of Washington, 1920.