§ 1. Congruences and their simplest properties § 2. Roots of congruences. Complete set of residues § 3. Roots of polynomials and roots of congruences § 4. Congruences of the first degree § 5. Wilson's theorem and the simple theorem of Fermat § 6. Numeri idonei § 7. Pseudoprime and absolutely pseudoprime numbers § 8. Lagrange's theorem § 9. Congruences of the second degree