Question

Show that $a^2 \bmod 24=1$ for each $a \perp 24$. Conclude that $a^{-1} \bmod 24=$ $a \bmod 24$ for each $a \perp 24$.

   Show that $a^2 \bmod 24=1$ for each $a \perp 24$. Conclude that $a^{-1} \bmod 24=$ $a \bmod 24$ for each $a \perp 24$.
Applied Algebra: Codes, Ciphers and Discrete Algorithms
Applied Algebra: Codes, Ciphers and Discrete Algorithms
Darel W. Hardy, Fred… 2nd Edition
Chapter 8, Problem 2 ↓

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Step 1

- $a \perp 24$ means that $a$ and $24$ are coprime, i.e., the greatest common divisor (gcd) of $a$ and $24$ is $1$. - $a^2 \bmod 24 = 1$ means that when $a^2$ is divided by $24$, the remainder is $1$. - We need to show that for every integer $a$ that is coprime to  Show more…

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Show that $a^2 \bmod 24=1$ for each $a \perp 24$. Conclude that $a^{-1} \bmod 24=$ $a \bmod 24$ for each $a \perp 24$.
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Key Concepts

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Modular Arithmetic
Modular arithmetic studies the properties of integers with respect to a modulus. It involves operations like addition, subtraction, multiplication, and determining congruences. In problems like this, the concept of reduction modulo a number (here, 24) is central, where two numbers are considered equivalent if they differ by a multiple of the modulus.
Coprimality
Coprimality refers to the condition when two numbers have no common factors other than 1. In this context, a and 24 being coprime (written as a ? 24) means that a is a unit modulo 24, which allows the existence of a multiplicative inverse. This property is foundational in modular arithmetic as it defines the set of numbers for which inversion is possible.
Units in Modular Arithmetic
The units modulo n, denoted by the group of integers coprime to n under multiplication modulo n, are the numbers that have a multiplicative inverse modulo n. In this question, since a is coprime to 24, it is an element of the group of units modulo 24. The structure and properties of this group, such as whether each element is self-inverse, are crucial concepts.
Multiplicative Inverse and Self-Inverse Elements
A multiplicative inverse modulo n is an integer b such that a·b ? 1 (mod n). When an element is its own inverse (i.e., a ? a^(-1) modulo n), it means that a^2 ? 1 (mod n). Showing that a^2 ? 1 for every a coprime to 24 demonstrates that every element in the group of units modulo 24 is self-inverse, a property that has interesting implications in the study of group theory and modular arithmetic.

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