Subspaces and Their Dimensions

Oct 13, 2025 by wbrum.org 98 views

Introduction

In the realm of linear algebra, understanding the properties and behavior of subspaces is crucial for solving various problems and theorems. One such problem, as presented in Axler's Linear Algebra Done Right 4th edition (problem #16 p. 49), involves proving the existence of subspaces whose intersection equals a given subspace. In this article, we will delve into the details of this problem and provide a comprehensive solution.

Problem Statement

Suppose 𝑉 is a finite-dimensional vector space and 𝑈 is a subspace of 𝑉 with 𝑈 ≠ 𝑉. Let 𝑛 = dim 𝑉 and 𝑚 = dim 𝑈. We need to prove that there exist subspaces 𝑊₁ and 𝑊₂ of 𝑉 such that 𝑊₁ ∩ 𝑊₂ = 𝑈.

Understanding the Problem

To approach this problem, we need to understand the properties of subspaces and their dimensions. The dimension of a subspace is defined as the number of vectors in a basis for that subspace. In this case, we are given that 𝑉 is finite-dimensional, meaning it has a finite basis. We are also given that 𝑈 is a subspace of 𝑉 with 𝑈 ≠ 𝑉, which implies that 𝑈 is a proper subspace of 𝑉.

Key Concepts

Before we proceed with the solution, let's recall some key concepts related to subspaces and their dimensions.

Subspace: A subspace of a vector space 𝑉 is a subset of 𝑉 that is closed under addition and scalar multiplication.
Dimension: The dimension of a subspace is defined as the number of vectors in a basis for that subspace.
Basis: A basis for a subspace is a set of linearly independent vectors that span the subspace.

Solution

To prove the existence of subspaces 𝑊₁ and 𝑊₂ of 𝑉 such that 𝑊₁ ∩ 𝑊₂ = 𝑈, we will use the following approach:

Step 1: Constructing a basis for 𝑈
- Let {𝑣₁, 𝑣₂, …, 𝑣𝑚} be a basis for 𝑈.
- Since 𝑈 ≠ 𝑉, there exists a vector 𝑣 ∈ 𝑉 such that 𝑣 ∉ 𝑈.
Step 2: Constructing a basis for 𝑊₁
- Let {𝑣₁, 𝑣₂, …, 𝑣𝑚, 𝑣} be a basis for 𝑊₁.
- Since {𝑣₁, 𝑣₂, …, 𝑣𝑚} is a basis for 𝑈, we have that 𝑊₁ ∩ 𝑈 = 𝑈.
Step 3: Constructing a basis for 𝑊₂
- Let {𝑣₁, 𝑣₂, …, 𝑣𝑚, 𝑣 + 𝑤} be a basis for 𝑊₂, where 𝑤 is a vector in 𝑉 such that 𝑤 ∉ 𝑈.
- Since {𝑣₁, 𝑣₂, …, 𝑣𝑚} is a basis for 𝑈, we have that 𝑊₂ ∩ 𝑈 = 𝑈.
Step 4: Verifying that 𝑊₁ ∩ 𝑊₂ = 𝑈
- We need to show that 𝑊₁ ∩ 𝑊₂ = 𝑈.
- Let 𝑥 ∈ 𝑊₁ ∩ 𝑊₂.
- Since 𝑥 ∈ 𝑊₁, we have that 𝑥 = 𝑎₁𝑣₁ + 𝑎₂𝑣₂ + … + 𝑎𝑚𝑣𝑚 + 𝑏𝑣 for some scalars 𝑎₁, 𝑎₂, …, 𝑎𝑚, 𝑏.
- Since 𝑥 ∈ 𝑊₂, we have that 𝑥 = 𝑎₁𝑣₁ + 𝑎₂𝑣₂ + … + 𝑎𝑚𝑣𝑚 + 𝑐(𝑣 + 𝑤) for some scalars 𝑎₁, 𝑎₂, …, 𝑎𝑚, 𝑐.
- Equating the two expressions for 𝑥, we get 𝑏𝑣 = 𝑐(𝑣 + 𝑤).
- Since 𝑣 ∉ 𝑈, we have that 𝑏 = 0.
- Therefore, 𝑥 = 𝑎₁𝑣₁ + 𝑎₂𝑣₂ + … + 𝑎𝑚𝑣𝑚.
- Since {𝑣₁, 𝑣₂, …, 𝑣𝑚} is a basis for 𝑈, we have that 𝑥 ∈ 𝑈.
- Therefore, 𝑊₁ ∩ 𝑊₂ ⊆ 𝑈.
- Since 𝑈 ⊆ 𝑊₁ ∩ 𝑊₂, we have that 𝑊₁ ∩ 𝑊₂ = 𝑈.

Conclusion

In this article, we have proved the existence of subspaces 𝑊₁ and 𝑊₂ of 𝑉 such that 𝑊₁ ∩ 𝑊₂ = 𝑈, where 𝑉 is a finite-dimensional vector space and 𝑈 is a proper subspace of 𝑉. We have used the concept of basis and dimension to construct the subspaces 𝑊₁ and 𝑊₂ and verified that their intersection equals 𝑈.

References

Axler, S. (2015). Linear Algebra Done Right. 4th edition. Springer.
Hoffman, K., & Kunze, R. (1971). Linear Algebra. 2nd edition. Prentice Hall.

Introduction

In our previous article, we proved the existence of subspaces 𝑊₁ and 𝑊₂ of a finite-dimensional vector space 𝑉 such that 𝑊₁ ∩ 𝑊₂ = 𝑈, where 𝑈 is a proper subspace of 𝑉. In this article, we will address some common questions and concerns related to this problem.

Q: What is the significance of the problem?

A: The problem is significant because it highlights the importance of understanding the properties of subspaces and their dimensions. The existence of subspaces whose intersection equals a given subspace has far-reaching implications in various areas of mathematics, including linear algebra, abstract algebra, and geometry.

Q: What are some common applications of this problem?

A: Some common applications of this problem include:

Linear Transformations: The problem is used to study the properties of linear transformations and their kernels.
Eigenvalues and Eigenvectors: The problem is used to study the properties of eigenvalues and eigenvectors of linear transformations.
Spectral Theory: The problem is used to study the properties of spectral theory and its applications to linear algebra.

Q: How does the problem relate to other areas of mathematics?

A: The problem is closely related to other areas of mathematics, including:

Abstract Algebra: The problem is used to study the properties of groups, rings, and fields.
Geometry: The problem is used to study the properties of geometric objects, such as points, lines, and planes.
Topology: The problem is used to study the properties of topological spaces and their invariants.

Q: What are some common mistakes to avoid when solving this problem?

A: Some common mistakes to avoid when solving this problem include:

Failing to check the dimension of the subspaces: It is essential to check the dimension of the subspaces to ensure that they are finite-dimensional.
Failing to verify the intersection of the subspaces: It is essential to verify that the intersection of the subspaces equals the given subspace.
Failing to use the correct basis for the subspaces: It is essential to use the correct basis for the subspaces to ensure that they are well-defined.

Q: How can I apply this problem to real-world scenarios?

A: This problem can be applied to real-world scenarios in various areas, including:

Computer Science: The problem can be used to study the properties of algorithms and data structures.
Engineering: The problem can be used to study the properties of mechanical systems and electrical circuits.
Economics: The problem can be used to study the properties of economic systems and their invariants.

Conclusion

In this article, we have addressed some common questions and concerns related to the existence of subspaces whose intersection equals a given subspace. We have highlighted the significance of the problem, its applications, and its relation to other areas of mathematics. We have also provided some common mistakes to avoid when solving this problem and discussed how it can be applied to real-world scenarios.

References

Axler, S. (2015). Linear Algebra Done Right. 4th edition. Springer.
Hoffman, K., & Kunze, R. (1971). Linear Algebra. 2nd edition. Prentice Hall.