**Course Level 1:** Introductory

**Estimated Study Time:** 18-20 hours

This course is a branch of mathematics concerned with the properties of integers. Section 1 introduces Euclid’s algorithm, which is used to find the HCF of two integers, and the idea of congruences, mathematical statements used to compare remainders when two integers are each divided by another integer. Section 2 introduces modular arithmetic, in which the usual arithmetic operations are applied to congruences. Section 3 introduces multiplicative inverses, which provide a method for division in modular arithmetic, and their use in solving linear congruences which are used in cryptography for disguising information or ciphers.

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