# 13 Pre-Class Assignment: Projections¶

## Readings for this topic (Recommended in bold)¶

## Goals for today’s pre-class assignment¶

## 1. Orthogonal and Orthonormal¶

**Definition:** A set of vectors is said to be **orthogonal** if every pair of vectors in the set is orthogonal (the dot product is 0).
The set is **orthonormal** if it is orthogonal and each vector is a unit vector (norm equals 1).

**Result:** An orthogonal set of nonzero vectors is linearly independent.

**Definition:** A basis that is an orthogonal set is called an orthogonal basis.
A basis that is an orthonormal set is called an orthonormal basis.

**Result:** Let \(\{u_1,\dots,u_n\}\) be an orthonormal basis for a vector space \(V\).
Then for any vector \(v\) in \(V\), we have
$\(v=(v\cdot u_1)u_1+(v\cdot u_2)u_2 +\dots + (v\cdot u_n)u_n\)$

**Definition:** A *square* matrix is **orthogonal** is \(A^{-1}=A^\top\).

**Result:** Let \(A\) be a square matrix. The following three statements are equivalent.

(a) \(A\) is orthogonal.

(b) The column vectors of \(A\) form an orthonormal set.

(c) The row vectors of \(A\) form an orthonormal set.

(d) \(A^{-1}\) is orthogonal.

(e) \(A^\top\) is orthogonal.

**Result:** If \(A\) is an orthogonal matrix, then we have \(|A|=\pm 1\).

Consider the following vectors \(u_1, u_2\), and \(u_3\) that form a basis for \(R^3\).

✅ **DO THIS:** Show that the vectors \(u_1\), \(u_2\), and \(u_3\) are linearly independent (**HINT:** see the pre-class for 0219-Change_Basis):

Put your answer to the above here

✅ **QUESTION 1:** How do you show that \(u_1\), \(u_2\), and \(u_3\) are orthogonal?

Put your answer to the above question here

✅ **QUESTION 2:** How do you show that \(u_1\), \(u_2\), and \(u_3\) are normal vectors?

Put your answer to the above question here

✅ **DO THIS:** Express the vector \(v = (7,5,-1)\) as a linear combination of the \(u_1\), \(u_2\), and \(u_3\) basis vectors:

```
# Put your answer here
```

## 2. Code Review¶

In the next in-class assignment, we are going to avoid some of the more advanced libraries ((i.e. no `numpy`

or `scipy`

or `sympy`

) to try to get a better understanding about what is going on in the math.
The following code implements some common linear algebra functions:

```
#Standard Python Libraries only
import math
import copy
```

```
def dot(u,v):
'''Calculate the dot product between vectors u and v'''
temp = 0;
for i in range(len(u)):
temp += u[i]*v[i]
return temp
```

✅ **DO THIS:** Write a quick test to compare the output of the above `dot`

function with the `numpy`

dot function.

```
# Put your test code here
```

```
def multiply(m1,m2):
'''Calculate the matrix multiplication between m1 and m2 represented as list-of-list.'''
n = len(m1)
d = len(m2)
m = len(m2[0])
if len(m1[0]) != d:
print("ERROR - inner dimentions not equal")
#make zero matrix
result = [[0 for j in range(m)] for i in range(n)]
# print(result)
for i in range(0,n):
for j in range(0,m):
for k in range(0,d):
#print(i,j,k)
#print('result', result[i][j])
#print('m1', m1[i][k])
#print('m2', m2[k][j])
result[i][j] = result[i][j] + m1[i][k] * m2[k][j]
return result
```

✅ **DO THIS:** Write a quick test to compare the output of the above `multiply`

function with the `numpy`

multiply function.

```
# Put your test code here
```

✅ **QUESTION:** What is the big-O complexity of the above `multiply`

function?

Put your answer to the above question here.

✅ **QUESTION:** Line 11 in the provided `multiply`

code initializes a matrix of the size of the output matrix as a list of lists with zeros. What is the big-O complexity of line 11?

Put your answer to the above question here.

```
def norm(u):
'''Calculate the norm of vector u'''
nm = 0
for i in range(len(u)):
nm += u[i]*u[i]
return math.sqrt(nm)
```

✅ **DO THIS:** Write a quick test to compare the outputs of the above `norm`

function with the `numpy`

norm function.

```
#Put your test code here
```

```
def transpose(A):
'''Calculate the transpose of matrix A represented as list of lists'''
n = len(A)
m = len(A[0])
AT = list()
for j in range(0,m):
temp = list()
for i in range(0,n):
temp.append(A[i][j])
AT.append(temp)
return AT
```

✅ **DO THIS:** Write a quick test to compare the output of the above `transpose`

function with the `numpy`

transpose function.

```
# Put your test code here
```

✅ **QUESTION:** What is the big-O complexity of the above `transpose`

function?

Put your answer to the above question here

✅ **QUESTION:** Explain any differences in results between the provided functions and their `numpy`

counterparts.

Put your answer to the above question here

## 3. Gram-Schmidt¶

Watch this video for the indroduction of Gram-Schmidt, which we will implement in class.

```
from IPython.display import YouTubeVideo
YouTubeVideo("rHonltF77zI",width=640,height=360, cc_load_policy=True)
```

## 4. Assignment wrap-up¶

✅ **Assignment-Specific QUESTION:** How do you show that \(u_1\), \(u_2\), and \(u_3\) are orthogonal?

Put your answer to the above question here

✅ **QUESTION:** Summarize what you did in this assignment.

Put your answer to the above question here

✅ **QUESTION:** What questions do you have, if any, about any of the topics discussed in this assignment after working through the jupyter notebook?

Put your answer to the above question here

✅ **QUESTION:** How well do you feel this assignment helped you to achieve a better understanding of the above mentioned topic(s)?

Put your answer to the above question here

✅ **QUESTION:** What was the **most** challenging part of this assignment for you?

Put your answer to the above question here

✅ **QUESTION:** What was the **least** challenging part of this assignment for you?

Put your answer to the above question here

✅ **QUESTION:** What kind of additional questions or support, if any, do you feel you need to have a better understanding of the content in this assignment?

Put your answer to the above question here

✅ **QUESTION:** Do you have any further questions or comments about this material, or anything else that’s going on in class?

Put your answer to the above question here

✅ **QUESTION:** Approximately how long did this pre-class assignment take?

Put your answer to the above question here

Written by Dr. Dirk Colbry, Michigan State University

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.