Chapter 9: Problem 12
Let \(I\) be a nontrivial ideal in \(\mathbb{Z}[i]\). Show that \(\mathbb{Z}[i] / I\) is a finite ring.
Short Answer
Expert verified
[31;1m123][饾憱]/饾懓鈥 is finite because it consists of cosets formed from a finite number of elements due to the [32;1mnt鈥 norm.
Step by step solution
01
Understanding Ideals in [31;1m[0m[31;1m [0m[饾憤][饾憱]
In the ring of Gaussian integers [31;1m[0m[31;1m鈥⑩潙峕[饾憱], an ideal [31;1m饾懓鈥 can consist of all integer combinations of a specific Gaussian integer [32;1m饾憥+饾憦饾憱鈥 . A nontrivial ideal [31;1m饾懓鈥 means [32;1m饾懓鈥 is not the zero ideal, and [32;1mhave鈥 includes at least one non-zero element.
02
Analyze Cosets of the Ideal
The ring [31;1m饾憤][饾憱]/饾懓鈥 is formed by the cosets of [32;1m饾懓鈥 . Each coset is of the form [32;1m饾憥+饾憦饾憱+饾懓鈥 where [34;1m饾憥+饾憦饾憱鈥 is a generator of[31;1m饾懓鈥 . Verify that distinct integers or cosets satisfy [34;1m饾憥+饾憦饾憱鈥 鈭 饾懓鈥 . Thus, the number of distinct cosets corresponds to positive integers [饾憤][饾憱]/饾憶鈥.
03
Determine Finiteness of Cosets
Each coset [31;1m/饾憱][饾憱]鈥 can be represented by varying [34;1m饾憥+饾憦饾憱鈥 from [32;1m饾懓鈥 . Thanks to the finite nature of multiples, these distinct combinations group comfortably in a finite tally of cosets.
04
Relate with Norm Function
Consider the norm, [34;1m饾憗(饾憥+饾憦饾憱)=饾憥虏+饾憦虏鈥 . If the norm of [31;1m饾憥+饾憦饾憱鈥 is [31;1m饾憶鈥 , any element in [32;1m饾懓鈥 must have its norm as a multiple of [31;1m饾憶鈥 . Since the norm function is a mapping of [32;1m饾憥+饾憦饾憱饾憸饾憽饾憻鈥 integers, it implies finitely many coset members with reduced form.
05
Conclude on Ring Finiteness
The number of coset representatives [31;1m ]/饾懓鈥 is finite. Therefore, [34;1m饾憤][饾憱]/饾懓鈥 is a finite ring.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Ideals
An ideal in the context of rings, including the ring of Gaussian integers \(\mathbb{Z}[i]\), is an important element. It consists of all integer combinations of a specific element from the ring. When we refer to a nontrivial ideal, we exclude the zero ideal, which contains only the number zero. Instead, we work with ideals that include at least one non-zero element. Ideals in \(\mathbb{Z}[i]\) maintain the essential property that, if you multiply any element from \(\mathbb{Z}[i]\) by an element of the ideal, the result will also belong to the ideal itself. This creates a stable structure where members are predictable by their defining properties.
Cosets
Cosets are fundamental in understanding quotient rings like \(\mathbb{Z}[i] / I\), where \(I\) is an ideal. In essence, cosets partition a ring into non-overlapping subsets. Each coset can be expressed in the form \(a + bi + I\), where \(a + bi\) is a representation of any member of \(\mathbb{Z}[i]\), and \(I\) is the ideal that helps form these coset families. Importantly, two elements belong to the same coset if their difference is in the ideal \(I\). For any ring directly related to cosets, if the number of cosets is countable, it implies certain finiteness in the properties of the overall ring structure. This finiteness assures us that the set of distinct cosets鈥攁nd hence the ring \(\mathbb{Z}[i] / I\)鈥攊s limited.
Gaussian Integers
Gaussian integers are numbers of the form \(a + bi\), where \(a\) and \(b\) are integers, and \(i\) represents the square root of \(-1\). They form a unique mathematical ring, denoted \(\mathbb{Z}[i]\). This ring behaves similarly to the integers \(\mathbb{Z}\) but extends into the complex plane. Properties of Gaussian integers include closure under addition, subtraction, and multiplication. They are foundational in establishing ideals and examining their behavior in the ring \(\mathbb{Z}[i]\). Gaussian integers help depict structures visually through lattice points in the complex plane, where each point corresponds to a Gaussian integer.
Norm Function
The norm function for Gaussian integers is defined as \(N(a + bi) = a^2 + b^2\). This mathematical operation maps a Gaussian integer to a non-negative integer. The norm is essential because it provides insights into the size and divisibility properties of Gaussian integers within the ring. When an ideal \(I\) in \(\mathbb{Z}[i]\) is described by a Gaussian integer with a particular norm \(n\), it implies that elements in \(I\) have norms that are multiples of \(n\). Besides confirming structural properties like finiteness, the norm function allows us to assess the uniqueness of cosets and their representatives. As a direct result, the norms defining these ideals suggest that there are finitely many reduced forms, further confirming the finite nature of \(\mathbb{Z}[i] / I\).