Chapter 1 Numbers

1.1 Discrete Numbers

In this module we will explore numbers and ways in which we use numbers. Through history we have slowly increased the different sorts of number that we have, so that we can model the world in which we live. At this link you can get a little bit of history about who invented various numbers

https://www.whoinventedit.net/who-invented-numbers.html

We start off with natural numbers

\[ {\mathbb N} = \{1,2,3,\cdots\}. \]

1.1.1 The cultural heritage of mathematics

1.2 Solving Equations

One way of thinking about how some of these numbers can arise is by thinking about solving equations. For instance we can see that the rational numbers might arise by us trying to solve \[ m x = n, \] where \(m,n \in {\mathbb N}\). In Linear Algebra we think about solving systems of equations of this type (you will have called them simultaneous equations).

In this module we will think about things like quadratic equations, for instance \[ x^2-2=0. \] In the next frame you will see how to plot the function \(y=x^2-2\) using computational tool R. This has become one of the main tools used in industry today, and you should consider developing it in your degree to help the range of jobs you can do.

f <- function(x) {
  x^2 - 2
}

g <-function(x){ 
  0*x
}

  x <- seq(0,3,0.1)
  plot(x,f(x),main="f(x)=x^2-2",
ylab="f(x)",
type="l",
col="blue",
)
  lines(x,g(x),col="red")

An important question is "How do we know there is a number which solves the equation \(y=x^2\)? One of the most amazing proofs in mathematics is that we can show that it cannot be a rational number, so if there is one, then it is not a fraction.

1.3 Mathematical induction

Induction is a method of proof where we use things that we know already to prove the next thing. Intuitively it is like having a set of dominoes lined up. Watch the video below to get another version of this explanation.

https://www.bing.com/videos/search?q=induction+dominoes&view=detail&mid=D524EACA53AE5BD88DD8D524EACA53AE5BD88DD8&FORM=VIRE

I will now use induction to prove:

This process is called mathematical induction and it is one of the greatest weapons in the armoury of justifying mathematical statements (we call proof).

1.4 Complex numbers

In order to solve differential equations later we are going to need some new numbers, the complex numbers. The idea for complex numbers goes back a long way in history:

https://en.wikipedia.org/wiki/Imaginary_number

They are an invaluable tool in the scientists tool bag, and we give a quick introduction here. The starting point is to have have a square root of \(-1\) because this allows us to solve the equation \[ x^2+1=0. \] Now you might say that this is just not possible and no such number “really” exists. In what sense does the solution of \(x^2-2\) exist? These are interesting philosophical questions. It exists because we have just defined it. This number is called \(i\). The engineers often call it \(j\) so don’t get upset if you see this in some texts. The number \(i\) is referred to as imaginary, but this term was originally used by Descartes in a disparaging way.

1.4.1 Arithmetic with complex numbers

When \(z_1=x_1+iy_1\) and \(z_2=x_2+iy_2\) the sum is \((x_1+x_2)+i(y_1+y_2)\). The product is \((x_1 x_2 - y_1 y_2)+i(x_1 y_2+x_2 y_1)\).

Example 1.1 Let \(n \in {\mathbb N}\). Then Let \(z_1=3+4i\) and \(z_2=-1+i\). Then \(z_1+z_2=(3+4i)+(-1+i)=(3-2)+i(4+1)=1+5i\). Also \(z_1 z_2=(3+4i)(-1+i)=((3)(-1)-(4)(1)+i((4)(-1)+(3)(1))=-7-i\).


Example 1.2 Compute the inverse of \(z=2-4i\). First we have \(|z|^2=2^2+(-4)^2=20\). Also \(\bar z = 2+4i\). Thus \[ z^{-1}={\bar z \over |z|^2}={2+4i \over 20}={1 \over 10}+{i \over 5}. \] You can check this by direct multiplication.

1.4.2 Test yourself

1.4.2.1 Arithmetic with complex numbers

In this problem you will test yourself with calculation of modulus, argument, multiplication by complex conjugate, given two complex numbers.

1.4.2.2 Division of complex numbers

In this question you will practice dividing by complex numbers. A special case of this is computing inverses.

1.5 Challenge yourself

  1. Show using the same method as above that \(\sqrt{5}\) is not rational.
  2. Prove by induction that \(\sum_{i=1}^n i^m = {n^{m+1} \over m+1} + q(n)\), where \(q\) is a polynomial of degree \(m-1\).
  3. Find out what a transcendental number is. Why is \(\pi\) a transcendental number?
  4. The Fibonacci numbers are given by the following formula \(F_1=1\), \(F_2=2\), \(F_n=F_{n-1}+F_{n-2}\), \(n \ge 3\). Write down the first 10 Fibonacci numbers. Prove by induction that \[ F_n = A \left ({1-\sqrt{5} \over 2} \right )^n + B \left ({1+\sqrt{5} \over 2} \right )^n, \] for some constants \(A\) and \(B\). You should use the fact that \[ {1 \pm \sqrt{5} \over 2} \] are the roots of the quadratic equation \(x^2-x-1=0\). Use the first two values in the sequence to calculate these constants. Check your forumla with the examples you computed.