Description
Problem 1
For PCA, from the perspective of maximizing variance, please show that the solution of φ to
maximize kXφk
2
2
, s.t. kφk2 = 1 is exactly the first column of U, where [U, S] = svd(XT X). (Note:
you need prove why it is optimal than any other reasonable combinations of Ui
, say φˆ = 0.8 ∗ U(:
, 1) + 0.6 ∗ U(:, 2) which also satisfies kφˆk2 = 1.)
Problem 2
Why might we prefer to minimize the sum of absolute residuals instead of the residual sum of
squares for some data sets? Recall clustering method K-means when calculating the centroid, it is
to take the mean value of the data-points belonging to the same cluster, so what about K-medians?
What is its advantage over of K-means? Please use a synthetic (toy) experiment to illustrate your
conclusion.
Problem 3
Let’s revisit Least Squares Problem: minimize
β
1
2
ky − Aβk
2
2
, where A ∈ R
n×p
.
1
1. Please show that if p > n, then vanilla solution (AT A)
−1AT y is not applicable any more.
2. Let’s assume A = [1, 2, 4; 1, 3, 5; 1, 7, 7; 1, 8, 9], y = [1; 2; 3; 4]. Please show via experiment results that Gradient Descent method will obtain the optimal solution with Linear Convergence
rate if the learning rate is fixed to be 1
σmax(AT A)
, and β0 = [0; 0; 0].
3. Now let’s consider ridge regression: minimize
β
1
2
ky − Aβk
2
2 +
λ
2
kβk
2
2
, where A, y, β0 remains the same as above while learning rate is fixed to be 1
λ+σmax(AT A)
where λ varies from
0.1, 1, 10, 100, 200, please show that Gradient Descent method with larger λ converges faster.
Problem 4
Please download the image from https://en.wikipedia.org/wiki/Lenna#/media/File:Lenna_
(test_image).png with dimension 512 × 512 × 3. Assume for each RGB channel data X, we have
[U, Σ, V ] = svd(X).
Please show each compression ratio and reconstruction image if we choose first
2, 5, 20, 50, 80, 100 components respectively. Also please determine the best component number to
obtain a good trade-off between data compression ratio and reconstruction image quality.
(Open
question, that is your solution will be accepted as long as it’s reasonable.)
2
