# [Solved] CFRM405 Homework 5

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1. Give an example of a 2 × 2 matrix that has no real eigenvectors. Justify your solution with intuition (without solving completely for the eigenvectors and eigenvalues).
2. Consider an n×p matrix A. Show that the number of linear independent rows is the same as the number of linearly independent columns.

Hint: Write A = CR where C is a matrix of the linearly independent columns of A. Why can we write A like this? Then consider the CR product in the “row” interpretation of matrix multiplication.

1. Let A be an m×n matrix (assume m > n). The full singular value factorization A = UΣV T contains more information than necessary to reconstruct A.
• What are the smallest matrices U˜, Σ and˜ V˜T such that U˜Σ˜V˜T = A?
• Let . That is, think about U from the full singular value factorization as a block matrix consisting of the matrix U˜ found in part (a) and the remaining (unneeded) columns Uˆ.

Find expressions for U˜TU˜ and U˜U˜T.

• Use the reduced singular value factorization obtained in part (a) to find an expression for the matrix H = A(ATA)−1AT. How many matrices must be inverted (diagonal and orthogonal matrices don’t count)?
1. Let x and y be vectors of m The least squares solution for a best-fit line for a plot of y versus x is

βˆ = (XTX)−1XTy

where

• Suppose you know the full singular value factorization X = UΣV T. Find an expression for βˆ in terms of U, Σ, and V . Hint: Only square matrices can be invertible.
• Repeat part (a) using the reduced singular value factorization X = U˜Σ˜V˜T.
1. Let X˜ be an m × n matrix (m > n) whose columns have sample mean zero, and let X˜ = U˜Σ˜V˜T be a reduced singular value factorization of X˜. The squared Mahalanobis distance to the point ˜xiT (the ith row of X˜) is

d2i = x˜iTSˆ−1x˜i

where = cov(X˜). Explain how to compute d2i without inverting a matrix.

1. (a) Suppose A = LU where L is lower triangular and U is upper triangular. Explain how you would solve the problem Ax = b using L, U, and the concepts of forward and backward substitution.

(b) Compute the LU factorization of

• 2 −1

−3 2

• 1 1

by hand using elimination matrices.

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[Solved] CFRM405 Homework 5
10 USD \$