15. Is it possible for a 4 × 4 matrix to be invertible when its columns do not span R*? Why or why not? 16. If an n x n matrix A is invertible, then the columns of AT are linearly independent. Explain why. 17. Can a square matrix with two identical columns be invert- ible? Why or why not?

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Number 15 through 28
### Linear Algebra: Advanced Problem Set

**15.** Is it possible for a \( 4 \times 4 \) matrix to be invertible when its columns do not span \( \mathbb{R}^4 \)? Why or why not?

**16.** If an \( n \times n \) matrix \( A \) is invertible, then the columns of \( A^T \) are linearly independent. Explain why.

**17.** Can a square matrix with two identical columns be invertible? Why or why not?

**18.** Can a square matrix with two identical rows be invertible? Why or why not?

**19.** If the columns of a \( 7 \times 7 \) matrix \( D \) are linearly independent, what can be said about the solutions of \( Dx = b \)? Why?

**20.** If \( A \) is a \( 5 \times 5 \) matrix and the equation \( Ax = b \) is consistent for every \( b \) in \( \mathbb{R}^5 \), is it possible that for some \( b \), the equation \( Ax = b \) has more than one solution? Why or why not?

**21.** If the equation \( C u = v \) has more than one solution for some \( v \) in \( \mathbb{R}^n \), can the columns of the \( n \times n \) matrix \( C \) span \( \mathbb{R}^n \)? Why or why not?

**22.** If \( n \times n \) matrices \( E \) and \( F \) have the property that \( EF = I \), then \( E \) and \( F \) commute. Explain why.

**23.** Assume that \( F \) is an \( n \times n \) matrix. If the equation \( F x = y \) is inconsistent for some \( y \) in \( \mathbb{R}^n \), what can you say about the equation \( F x = 0 \)? Why?

**24.** If an \( n \times n \) matrix \( G \) cannot be row reduced to \( I_n \), what can you say about the columns of \( G \)? Why?

**25.** Verify the boxed statement preceding Example 1.

**
Transcribed Image Text:### Linear Algebra: Advanced Problem Set **15.** Is it possible for a \( 4 \times 4 \) matrix to be invertible when its columns do not span \( \mathbb{R}^4 \)? Why or why not? **16.** If an \( n \times n \) matrix \( A \) is invertible, then the columns of \( A^T \) are linearly independent. Explain why. **17.** Can a square matrix with two identical columns be invertible? Why or why not? **18.** Can a square matrix with two identical rows be invertible? Why or why not? **19.** If the columns of a \( 7 \times 7 \) matrix \( D \) are linearly independent, what can be said about the solutions of \( Dx = b \)? Why? **20.** If \( A \) is a \( 5 \times 5 \) matrix and the equation \( Ax = b \) is consistent for every \( b \) in \( \mathbb{R}^5 \), is it possible that for some \( b \), the equation \( Ax = b \) has more than one solution? Why or why not? **21.** If the equation \( C u = v \) has more than one solution for some \( v \) in \( \mathbb{R}^n \), can the columns of the \( n \times n \) matrix \( C \) span \( \mathbb{R}^n \)? Why or why not? **22.** If \( n \times n \) matrices \( E \) and \( F \) have the property that \( EF = I \), then \( E \) and \( F \) commute. Explain why. **23.** Assume that \( F \) is an \( n \times n \) matrix. If the equation \( F x = y \) is inconsistent for some \( y \) in \( \mathbb{R}^n \), what can you say about the equation \( F x = 0 \)? Why? **24.** If an \( n \times n \) matrix \( G \) cannot be row reduced to \( I_n \), what can you say about the columns of \( G \)? Why? **25.** Verify the boxed statement preceding Example 1. **
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