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# ECE421 Assignment 1 solved

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## Problem 1 – Clustering with κ-means

We will consider the UCI ML Breast Cancer
Wisconsin dataset. Features are computed from a digitized image of a fine needle aspirate
(FNA) of a breast mass. They describe characteristics of the cell nuclei present in the image.
You can download the dataset using the function:

Unless specified otherwise, questions in Problem 1 below refer to the UCI ML Breast Cancer
Wisconsin dataset when the word “dataset” is used.
1. (5 points) Implement κ-means yourself. Your function should take in an array containing
a dataset and a value of κ, and return the cluster centroids along with the cluster
assignment for each data point. You may choose the initialization heuristic of your
choice among the two we saw in class. Hand-in the code for full credit. For this
question, you should not rely on any library other than numPy in Python.

2. (1 point) Run the κ-means algorithm for values of κ varying between 2 and 7, at increments of 1. Justify in your answer which data you passed as the input to the κ-means
algorithm.
3. (2 points) Plot the distortion achieved by κ-means for values of κ varying between 2
and 7, at increments of 1. Hand-in the code and figure output for full credit. For this
question, you may rely on plotting libraries such as matplotlib.
4. (1 point) If you had to pick one value of κ, which value would you pick? Justify your
choice.

## Problem 2 – Lack of optimality of κ-means

1. (3 points) Construct an analytical demonstration that κ-means might converge to a
solution that is not globally optimal. Hint: consider the case where κ = 2 and the dataset
is made up of 4 points in R as follows: x(1) = 1, x(2) = 2, x(3) = 3, x(4) = 4. Initialize
κ-means with the centroids µ1 = 2 and µ2 = 4.

Note: you may assume that if a point
x(i) is equally distant to multiple centroids µk, the point will be assigned to the centroid
whose index is smallest, i.e., k with the smallest value for k ∈ arg mink �x(i) − µk�2.
Problem 3 – Linear algebra. If you are not familiar with the following linear algebra
concepts, you are highly encouraged to attend the corresponding tutorial session.

1. (1 point) Compute the matrix products AB, if possible, where
A :=

2 4 6
0 −2 4

, B :=

3 −2 5
0 8 2

2. (1 point) Compute the matrix products AB, if possible, where
A :=

2 4 6
0 −2 4

, B :=

2 −0.5
1 0
1 0.5

3. (3 points) Consider the following matrix:
A =

4 0 −2
1 3 −2
1 2 −1

Compute the characteristic polynomial of A and determine its eigenvalues. Hint: one of
the eigenvalues of the matrix is λ0 = 1.

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