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Let \(A\) be a \(3 \times 2\) matrix with rank \(2 .\) Give geometric descriptions of \(R(A)\) and \(N\left(A^{T}\right),\) and describe geometrically how the subspaces are related.

Short Answer

Expert verified
In this case, A is a $3 \times 2$ matrix with rank 2. R(A) is the column space, which is a two-dimensional subspace (a plane) in 鈩漗3, containing all linear combinations of the columns of A and passing through the origin. N(A^T) is the left nullspace, which is a one-dimensional subspace (a line) in 鈩漗3, containing all vectors orthogonal to the columns of A and passing through the origin. The relationship between these subspaces is that the line N(A^T) is orthogonal (perpendicular) to the plane R(A).

Step by step solution

01

Describe the column space (R(A))

The column space, also known as the range or R(A), is the set of all linear combinations of the columns of A. In this case, A is a 3x2 matrix with rank 2. This means that there are two linearly independent columns in A. Since there are two linearly independent columns, R(A) is a two-dimensional subspace in a 3-dimensional space (鈩漗3). Geometrically, R(A) can be described as a plane that passes through the origin in 鈩漗3.
02

Describe the left nullspace (N(A^T))

The left nullspace, or N(A^T), is the set of all vectors x in 鈩漗3 such that A^T x = 0. Since the rank of A is 2 and the matrix is 2x3 after transposition, the dimension of N(A^T) will be equal to the difference between the number of rows and the rank of the matrix (dim(N(A^T)) = 3 - 2 = 1). N(A^T) is a one-dimensional subspace, and geometrically, it can be described as a line passing through the origin in 鈩漗3.
03

Describe the relationship between R(A) and N(A^T)

Now, we need to describe how the subspaces R(A) and N(A^T) are geometrically related. The relationship between these subspaces can be explained using the Dimension Theorem: dim(R(A)) + dim(N(A^T)) = dim(鈩漗3) Since R(A) is a two-dimensional subspace (a plane) and N(A^T) is a one-dimensional subspace (a line) in 鈩漗3, their sum equals the dimension of 鈩漗3. Geometrically, the line N(A^T) is orthogonal (perpendicular) to the plane R(A). This means that the vectors in the left nullspace of A are orthogonal to the vectors in the column space of A.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Column Space
Understanding the column space is fundamental in linear algebra. It is the set of all possible linear combinations of the matrix's columns. For a given matrix A, the column space, denoted by R(A), represents a subspace within the larger vector space that A is part of.

In the context of a 3 x 2 matrix with a rank of 2, the column space is spanned by the matrix's two columns since they are said to be linearly independent. This means any vector in R(A) can be written as a combination of these columns. The geometric interpretation of R(A) is a plane that extends infinitely in all directions but remains flat and passes through the origin of 鈩漗3. This two-dimensional plane captures the essence of the column space and the constraints it imposes on the system it represents.
Left Nullspace
The left nullspace, sometimes known as the nullspace of A's transpose (N(A^T)), is slightly less intuitive but equally important. It is the set of all vectors that, when multiplied by A^T, result in the zero vector.

In the case of our 3 x 2 matrix with the rank of 2, after transposing, we have a 2 x 3 matrix. Due to the matrix's rank, we determine the left nullspace to be of dimension 1, indicating that it is a line. This line goes through the origin in 鈩漗3 and represents all the vectors orthogonal to every vector in the column space of A. To conceptualize it, visualize a straight line that intersects the previously mentioned plane at a right angle.
Dimension Theorem
The Dimension Theorem for matrices plays a pivotal role in linking different subspaces associated with a matrix. It gives us an elegant way to understand the structure of a matrix by relating its rank to the dimensions of its nullspace and column space.

The theorem states that the dimension of the column space plus the dimension of the nullspace equals the number of columns of the matrix. For the 3 x 2 matrix in question, we can write the equation as follows: \(\text{dim}(R(A)) + \text{dim}(N(A^T)) = \text{dim}(鈩漗3)\), which simplifies to \(2 + 1 = 3\). This balance is a fundamental property of linear systems, ensuring that all vectors in the larger vector space can be accounted for by these two subspaces.
Rank of Matrix
The rank of a matrix is defined by the maximum number of linearly independent column vectors (or row vectors) within the matrix. This concept is not only abstract but serves practical purposes in determining the solutions to a system of linear equations represented by the matrix.

For a matrix A of 3 x 2 with rank 2, this indicates that both column vectors contribute unique information to the system. There is no redundancy, and each column adds a new dimension to the column space. In terms of applications, a full-rank matrix like this often corresponds to a system of equations that has a unique solution, barring over-determined or under-determined scenarios.
Subspaces in Linear Algebra
Subspaces in linear algebra are essentially smaller vector spaces nested within a larger vector space that adhere to two main operations - vector addition and scalar multiplication. For subspaces associated with matrices, this often includes the column space and nullspaces, among others.

In the exercise at hand, both R(A) and N(A^T) are subspaces of 鈩漗3. These subspaces have striking geometric representations and crucial algebraic implications. The interplay between subspaces, as observed with R(A) being a plane and N(A^T) being a line orthogonal to it, showcases the beauty of linear algebra - where algebra meets geometry. These relationships are not just abstract representations but are the foundation for solving linear equations and understanding the behavior of linear transformations.

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Most popular questions from this chapter

Let \(A\) be an \(m \times n\) matrix, \(B\) an \(n \times r\) matrix, and \(C=A B .\) Show that (a) \(N(B)\) is a subspace of \(N(C)\) (b) \(N(C)^{\perp}\) is a subspace of \(N(B)^{\perp}\) and, consequently, \(R\left(C^{T}\right)\) is a subspace of \(R\left(B^{T}\right)\)

Let \(\mathbf{x}\) and \(\mathbf{y}\) be linearly independent vectors in \(\mathbb{R}^{n}\) and let \(S=\operatorname{Span}(\mathbf{x}, \mathbf{y}) .\) We can use \(\mathbf{x}\) and \(\mathbf{y}\) to define a matrix \(A\) by setting $$A=\mathbf{x y}^{T}+\mathbf{y} \mathbf{x}^{T}$$ (a) Show that \(A\) is symmetric. (b) Show that \(N(A)=S^{\perp}\) (c) Show that the rank of \(A\) must be 2

Given \(\mathbf{x}_{1}=\frac{1}{2}(1,1,1,-1)^{T}\) and \(\mathbf{x}_{2}=\frac{1}{6}(1,1,3,5)^{T}\) verify that these vectors form an orthonormal set in \(\mathbb{R}^{4} .\) Extend this set to an orthonormal basis for \(\mathbb{R}^{4}\) by finding an orthonormal basis for the null space of $$\left(\begin{array}{rrrr} 1 & 1 & 1 & -1 \\ 1 & 1 & 3 & 5 \end{array}\right)$$ [Hint: First find a basis for the null space and then use the Gram-Schmidt process.

\(\operatorname{Let}\left\\{\mathbf{x}_{1}, \mathbf{x}_{2}, \ldots, \mathbf{x}_{k}, \mathbf{x}_{k+1}, \ldots, \mathbf{x}_{n}\right\\}\) be an orthonormal basis for an inner product space \(V\). Let \(S_{1}\) be the subspace of \(V\) spanned by \(\mathbf{x}_{1}, \ldots, \mathbf{x}_{k},\) and let \(S_{2}\) be the subspace spanned by \(\mathbf{x}_{k+1}, \mathbf{x}_{k+2}, \ldots, \mathbf{x}_{n} .\) Show that \(S_{1} \perp S_{2}\)

Use the zeros of the Legendre polynomial \(P_{2}(x)\) to obtain a two-point quadrature formula $$\int_{-1}^{1} f(x) d x \approx A_{1} f\left(x_{1}\right)+A_{2} f\left(x_{2}\right)$$

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