Linear Algebra: Range and Null Space

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Questions and Answers

What is the set of all vectors in $\mathbb{R}^{m}$ that can be written as linear combinations of the columns of $A$?

  • $R(A)$ (correct)
  • $\mathbb{R}^{n}$
  • $N(A)$
  • $N(A^T)$

Which of the following statements is true about the rank of matrix $A$?

  • The rank is denoted by $r$ and $r \geq \max(m, n)$.
  • The rank is always equal to $n$.
  • The rank is always equal to $m$.
  • The rank is denoted by $r$ and $r \leq \min(m, n)$. (correct)

If $A \in \mathbb{R}^{m \times n}$, what is the relationship between $N(A)$ and $\mathbb{R}^{n}$?

  • $N(A)$ is equal to $\mathbb{R}^{m}$.
  • $N(A)$ is a subset of $\mathbb{R}^{m}$.
  • $N(A)$ is equal to $\mathbb{R}^{n}$.
  • $N(A)$ is a subset of $\mathbb{R}^{n}$. (correct)

Which of the following correctly describes $N(A)$?

<p>It is the set of all vectors $x \in \mathbb{R}^{n}$ such that $Ax = 0$. (C)</p> Signup and view all the answers

If $A \in \mathbb{R}^{m \times n}$ and its transpose $A^T \in \mathbb{R}^{n \times m}$, what is $R(A^T)$?

<p>The span of the columns of $A^T$. (C)</p> Signup and view all the answers

According to the property $\dim(N(A)) + \dim(R(A)) = n$, what does this imply?

<p>The dimension of the null space and range space of $A$ add up to $n$. (A)</p> Signup and view all the answers

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Study Notes

Range Space, R(A)

  • The range space of a matrix A is the set of all vectors in R^m that can be written as linear combinations of the columns of A.
  • Formally, R(A) = span({a_1, a_2,..., a_n}) = {y = Ax, x ∈ R^n, y ∈ R^m}
  • Similarly, R(A^T) = span({a_1, a_2,..., a_m}) = {w = A^Tz, z ∈ R^m, w ∈ R^n}

Properties of Range Space

  • R(A) ⊂ R^m
  • dim(R(A)) or dim(R(A^T)) is the rank of A, denoted by r, and r ≤ min(m, n)

Null Space, N(A)

  • The null space of a matrix A is the set of all vectors x mapped into zero by A.
  • N(A) = {x: Ax = 0, x ∈ R^n}
  • Similarly, N(A^T) = {z: Az = 0, z ∈ R^m}

Properties of Null Space

  • N(A) ⊂ R^n
  • dim(N(A)) + dim(R(A)) = n

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