§ 1. Number of divisors § 2. Sums d(1) + d(2) + … + d(n) § 3. Numbers d(n) as coefficients of expansions § 4. Sum of divisors § 5. Perfect numbers § 6. Amicable numbers § 7. The sum σ(1) + σ(2) + … + σ(n) § 8. The numbers σ(n) as coefficients of various expansions § 9. Sums of summands depending on the natural divisors of a natural number n § 10. Möbius function § 11. Liouville function λ(n)