9. Lecture – 57: Properties of GCD and Bezout’s Theorem
The chapter discusses properties of the greatest common divisor (GCD) and Bezout’s theorem, emphasizing the expressibility of GCD as a linear combination of two integers. The extended Euclidean algorithm is introduced for determining GCD and finding Bezout's coefficients, which are crucial for calculating modular multiplicative inverses. Additionally, the conditions for the existence of modular inverses are outlined, focusing on coprimality between integers and their modulus.
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What we have learnt
- Bezout's theorem states that the GCD of two integers can be expressed as a linear combination of those integers.
- The extended Euclidean algorithm not only computes the GCD but also finds integer coefficients that can express the GCD as such a linear combination.
- A multiplicative inverse of an integer modulo N exists if and only if that integer is coprime to N.
Key Concepts
- -- Bezout's Theorem
- A theorem stating that for any two integers a and b, there exist integers s and t such that GCD(a, b) = sa + tb.
- -- Extended Euclidean Algorithm
- An extension of the Euclidean algorithm that computes not only the GCD of two integers but also finds integers s and t that satisfy Bezout's identity.
- -- Coprimality
- Two integers are coprime if their greatest common divisor is 1, indicating that they share no common positive divisors other than 1.
- -- Multiplicative Inverse Modulo N
- An integer b is the multiplicative inverse of a modulo N if (a * b) mod N = 1, indicating that b undoes the multiplication of a in modulo N arithmetic.
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