# MU123 Unit 3 Notes

# 1 Natural numbers

Positive integers `1, 2, 3, 4...`

Some include zero, some not.

## 1.1 Multiples

A result of multiplying a natural number by natural number. E.g. multiples of 8 are $8, 16, 24, 32$

Thus multiples are numbers into which 8 divides exactly.

### Common multiple

is a multiple of all the numbers in question. Lowest common multiple (LCM) is the smallest number that is a multiple of numbers in question

## 1.2 Factors

A natural number that divides a number exactly into a second natural number, e.g. 2 is factor of 10. Alternatively we can say 10 is divisible by 2.

Related to multiples since 2 is a factor of 10 is same as 10 is a multiple of 2.

All natural numbers > 1 have at least two factors - itself and 1.

Factors can be arranged into factor pairs - $1,10$; $2,5$.

Strategy to find factors of a number

- Try 1,2,3,4... in turn. When factor is found, write it down with the other factor in the pair
- Stop when you reach already found factor pair

### Common factors

is a factor of all the numbers in question. The highest common factor (HCF) is the largest number that is a factor of numbers in question.

## 1.3 Prime numbers

Natural numbers that have **exactly** two factors - 1 and itself, e.g. $2, 3, 5, 7, 11, 13, \ldots$.

### Sieve of Eratosthenes

An algorithm for finding all the prime numbers up to a certain number.

- Create a range of $2..n$ where $n$ is a certain number
- Mark all the numbers that are multiples of 2 and are $>= 2^2$
- Move to next unmarked number - 3 and mark its multiples that are $>= 3^2$
- ...
- All the unmarked numbers are primes

Python impelementation src:

```
def SieveOfEratosthenes(n):
# Create a boolean array
# "prime[0..n]" and initialize
# all entries it as true.
# A value in prime[i] will
# finally be false if i is
# Not a prime, else true.
prime = [True for i in range(n+1)]
p = 2
while (p * p <= n):
# If prime[p] is not
# changed, then it is a prime
if (prime[p] == True):
# Update all multiples of p
for i in range(p * p, n+1, p):
prime[i] = False
p += 1
# Print all prime numbers
for p in range(2, n+1):
if prime[p]:
print p,
```

Resources

- https://www.geeksforgeeks.org/sieve-of-eratosthenes/
- https://cp-algorithms.com/algebra/sieve-of-eratosthenes.html
- https://www.baeldung.com/cs/sieve-of-eratosthenes

### Mersenne primes

$$ 2^n - 1 ~\text{for natural number} ~n $$ It can be proven that if $2^n - 1$ is prime then $n$ must be a prime.

Resources

- https://en.wikipedia.org/wiki/Mersenne_prime
- https://www.mersenne.org/
- https://primes.utm.edu/mersenne/

## 1.4 Prime factors

A natural number > 1 that is not prime is called a **composite number**, e.g.: $4, 6, 8, 9, 10\ldots$.
A composite number can often be written as a product of even more factors forming a **factor tree**:

Hence we can say $360 = 2^3 \times 3^2 \times 5$. All of the factors here are primes.

I could've started with different nodes, e.g. $360 = 10 \times 36$, but the result would've been the same.

The process of writing a natural number as a product of factors > 1 is called **factorisation**.

The fundamental theorem of arithmeticEvery natural number > 1 can be written as product of prime numbers in

just one way.

The **prime factorisation** of a natural number is the product of prime factors that is equal to it, e.g.:
$$
2 = 2 \qquad 3 = 3 \qquad 4 = 2^2 \\
5 = 5 \qquad 6 = 2 \times 3 \qquad 7 = 7 \\
8 = 2^3 \qquad 9 = 3^2 \qquad 10 = 2 \times 5
$$

Because of the fundamental theorem of aithmetic we can say that prime numbers are the **building blocks of natural numbers**.

For prime factorisation it may be helpful to be systematic and follow: $$ \text{the smallest possible prime} \times \text{a number} $$ And iterate for each composite number in the factor tree, e.g.:

There's no other way to find prime factors of a number $n$ then trying all the primes < $n$. So multiplying two large primes is a quick process but expensive to reverse. This fact is a base to secure encryption systems.

### Finding LCM and HCF using prime factors

- Find the prime factorisation of the numbers.
- To find the LCM, multiply together the highest ower of each prime factor occuring in any of the numbers.
- To find the HCF, multiply together the lowest power of each prime factor common to all the numbers.

## 1.5 Powers

### Index laws

So far I've used these $$ \begin{aligned} a^m \times a^n &= a^{m+n} \qquad \frac{a^m}{a^n} = a^{m-n} \\ (a^m)^n &= a^{mn} \\ (a \times b)^n &= a^n \times b^n \qquad (\frac{a}{b})^n = \frac{a^n}{b^n} \end{aligned} $$

# 2 Rational numbers

Include natural numbers among others.

Is a number that can be written as a fraction of two integers: $\frac{integer}{integer}$.

Following numbers can be written in this form:

- Fraction as $\frac{3}{4}$
- Mixed number as $5\frac{3}{4}$ since it can be converted to top-heavy fraction: $\frac{23}{4}$
- Whole number: $7 = \frac{7}{1}$
- Decimal number with finite number of digits after decimal points: $0.42 = \frac{42}{100}$
- Some decimal numbers with infinite numbers after decimal point: $0.333\ldots = \frac{1}{3}$
- The negative of any above: $-4 = \frac{-4}{1}$

### Irrational numbers

- $\pi$
- $\sqrt{2}$

### Rational numbers as decimals

Every rational number can be converted to decimal, e.g.: $\frac{5}{8} = 5 \div 8 = 0.625$ There are two possible outcomes of the division:

- decimal number with finite digits after d.p. -
**terminating decimal**- $0.625$ is terminating

- decimal with block of indefinitely repeating digits after d.p -
**recurring decimal**- $\frac{2}{3} = 0.6666\ldots$ is recurring
- Can be denoted with $0.1\overline{23}$.

**Thus rational numbers can be redefined as**

Decimal numbers that are terminating or recurring.

I can convert terminating to fraction so I can prove first part of the statement but I can't prove the second yet.

## 2.2 Adding and subtracting fractions

- Only when there's a same denominator - fractions are of a same type.
- When there's different denominator - find common one (LCM).
- Always simplify.

## 2.3 Multiplying and dividing fractions

### Multiplying

- Multiply the numerators together and multiply the denominators together.

### Dividing

- Multiple the fraction by its reciprocal

#### Reciprocal

A number and its reciprocal multiply together to give 1. E.g.: $0.25 ~\text{is rec of} ~4$,since $0.25 \times 4 = 1$ $\frac{3}{2} ~\text{is rec of} ~\frac{2}{3}$,since $\frac{3}{2} \times \frac{2}{3} = 1$

Alternatively, it is 1 divided by given number.

To find a reciprocal of fraction turn it upside down. E.g.: $\text{rec of}~\frac{3}{4}~\text{is}~\frac{4}{3}$

## 2.4 Negative indices

The previous positive power of a number $n$ is equal to current positive power divided by $n$, e.g.: $4^3 = \frac{4^4}{4}$

Thus $4^0$ has to be a number I get when dividing $\frac{4^1}{4} = 1$. Similarly $4^{-1}$ has to be a number I get when dividing $\frac{4^0}{4} = \frac{1}{4}$.

We can build whole scale of the pattern

$4^{-3}$ | $4^{-2}$ | $4^{-1}$ | $4^0$ | $4^1$ | $4^2$ | $4^3$ |
---|---|---|---|---|---|---|

$\frac{1}{4^3}$ | $\frac{1}{4^2}$ | $\frac{1}{4}$ | $1$ | $4$ | $16$ | $64$ |

The meaning of negative and zero indices works with all the index laws I've seen so far.

Index laws can be expand by following rules based on previous observations:

- A non-zero number raised to the power zero is 1: $$ a^0 = 1 $$
- A non-zero number raised to a negative power is the reciprocal of the number raised to the corresponding positive power: $$ a^{n - 1} = \frac{1}{a^n} $$

Interestingly: $$ a^{-1} = \frac{1}{a} $$ Hence, raising a number to the power of $-1$ is the same as finding its reciprocal.

E.g.: $$ (\frac{2}{3})^{-1} = 1 \times \frac{3}{2} = \frac{3}{2} $$

## 2.5 Scientific notation

- Also called
**standard form**. - A number between 1 and 10 (excluded) multiplied by a power of 10.

# 3 Irrational numbers and real numbers

## 3.1 What is an irrational number?

Decimals with infinite number of digits afterthe decimal point but no repeating block of digits.

Following number is not terminating nor recurring: $$ 0.010,010,001,000,010,000,01\ldots $$

*TODO: What if the repeating block is just really big?*

Here $x$ is not rational:

$$ x^2 = 2 \\ \sqrt{2} = x $$

The fact that $\sqrt{2}$ is irrational can be proven by **contradiction** followingly.

Suppose there's a rational number whose square is $2$. Hence it can be written as $$ \frac{\text{integer}}{\text{integer}} $$ Consider it being cancelled down to the simplest form as: $$ \frac{m}{n} $$ So $m$ and $n$ don't have any common factor. The square of $\frac{m}{n}$ is 2, so: $$ (\frac{m}{n})^2 = 2 \\ \frac{m^2}{n^2} = 2 $$ It follow that $$ \frac{m^2}{n^2} \times n^2 = 2 \times n^2 \\ m^2 = 2n^2 $$ $2n^2$ has to be even since it's a product of 2 and whole number. Meaning $m^2$ has to be even too, that means $m$ has to be even as well. That means that $m = 2r$ for some integer $r$. When substituting to the original equation we get: $$ (2r)^2 = 2n^2 \\ 4r^2 = 2n^2 \\ $$ It follow that $$ \frac{4r^2}{2} = \frac{2n^2}{2} \\ 2r^2 = n^2 $$

The integer $n^2$ is even hence $n$ has to be even too.
So both $m$ and $n$ are even. But that's **impossible** since $m$ and $n$ don't have any common factor.

The irrational numbers together with the rational numbers form the **real numbers**.

I can think about various categories of numbers as layers:

```
Real(
Irrational,
Rational(
Integers(
Natural
)
)
)
```

plus complex, that will come later on.

## 3.2 Roots of numbers

A number that when multiplied by itself gives the original number. Every positive number has two square roots - a positive one and a negative one.

Square roots of two numbers can be used to find the square root of the product or a quotient.

For example, numbers 16 and 49 have square roots of 4 and 7. I can apply index law $(a \times b)^n = a^n \times b^n$ to those: $$ (4 \times 7)^2 = 4^2 \times 7^2 \\ (4 \times 7)^2 = 16 \times 49 $$ That implies: $$ \sqrt{16 \times 49} = \sqrt{16} \times \sqrt{49} $$

This can be generalized into following laws: $$ \sqrt{a \times b} = \sqrt{a} \times \sqrt{b} \qquad \sqrt{\frac{a}{b}} = \frac{\sqrt{a}}{\sqrt{b}} $$

Obviously, these laws apply to any root.

## 3.3 Surds

Numbers that are not perfect square have irrational roots. Hence following roots are irrational:
$$
\sqrt{7}, \sqrt{8}, \sqrt{10}, \sqrt{11}
$$
Since such number can be only approximated as a decimal it should be left as is in calculations, e.g.:
$$
x = 2 \times \sqrt{5}
$$
such expressions are called **surds** - it contains one or more irrational roots of numbers.
Surds are usually written concisely as formulas, number before surd when multiplied:
$$
y = 2\sqrt{5}
$$
In the simplest form:
$$
\sqrt{12} = \sqrt{4 \times 3} = \sqrt{4} \times \sqrt{3} = 2\sqrt{3}
$$
This is obviously possible when some of a numbers under the root are perfect squares. Thus the goal when simplifying is to find some perfect square multiplied some other number giving the original number bellow the root.

### Multiplying roots

- Multiply numbers together
- Multiply roots together
- Simplify the root

### Dividing roots

Use following laws $$ \sqrt{a \times b} = \sqrt{a} \times \sqrt{b} \qquad \sqrt{\frac{a}{b}} = \frac{\sqrt{a}}{\sqrt{b}} $$

Sometimes, I should expand an integer to a multiplication of surds in order to simplify the quotient: $$ \frac{2}{\sqrt{2}} = \frac{\sqrt{2} \times \sqrt{2}}{\sqrt{2}} = \frac{1 \times \sqrt{2}}{1} = \sqrt{2} $$

### Adding and substracting roots

It's usually not possible to add or substract different roots, thus $$ \sqrt{a} + \sqrt{b} \not= \sqrt{a + b} $$

I can only add or substract roots that are the same.

To simplify surds

- Simplify roots of integers with square factors.
- Simplify products and quotients of roots.
- Add or subtract roots that are the same.

## 3.4 Fractional indices

Index laws can be applied to fractional indices too, e.g. let's consider the power $5^\frac{1}{2}$

By applying $$ a^m \times a^n = a ^{m + n} $$ I get $$ 5^\frac{1}{2} \times 5^\frac{1}{2} = 5^{\frac{1}{2} + \frac{1}{2}} = 5^1 = 5 $$ Hence $$ 5^\frac{1}{2} = \sqrt{5} $$

It can be generalized in the following index law $$ a^\frac{1}{n} = \sqrt[n]{a} $$

This rule toegher with other index laws can be used to give a meaning to any fractional index.

E.g. in combination with $$ (a^m)^n = a^{mn} $$

I can do following operation $$ 5^\frac{4}{3} = 5^{\frac{1}{3} \times 4} = (5^{\frac{1}{3}})^4 = (\sqrt[3]{5})^4 $$

This combination can be generalized to yet another index law $$ a^\frac{m}{n} = (\sqrt[n]{a})^m $$

Interestingly the square root laws I saw before: $$ \sqrt{a \times b} = \sqrt{a} \times \sqrt{b} \qquad \sqrt{\frac{a}{b}} = \frac{\sqrt{a}}{\sqrt{b}} $$ Can be deduced by applying $n = \frac{1}{2}$ to following index laws: $$ (a \times b)^n = a^n \times b^n \qquad (\frac{a}{b})^n = \frac{a^n}{b^n} $$

Irrational indices are also valid but I will encounter those later on.

# 4 Ratios

## 4.1 What is a ratio?

Comparition of multiple ordered quantities, e.g.: $3:5 ~ 2:10:7$

Sometimes it's useful to convert a ratio to a $number:1$ form to e.g.

- Compare multiple ratios.
- Find approximate ratios. To reach such a form I can divide both numbers by second number, e.g.: $$ 7 : 11 = \frac{7}{11} : \frac{11}{11} = 0.636 : 1 ~\text{(3 d.p.)} $$