# 0.1: Introduction

- Page ID
- 7440

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Mathematical objects come into existence by definitions. These definitions must give an absolutely clear picture of the object or concept. We don't need to prove them, simply to clearly define them. We are going to state some basic facts that needed in this course:

**Basic facts:**

- The collection of
**counting numbers**otherwise known as the collection of**natural numbers**is usually denoted by \(\mathbb{N}.\) We write \(\mathbb{N} = \{ 1,2,3,4, \dots\}.\) - The collection of the
**integers**is usually denoted by \(\mathbb{Z}\) and we write \({\mathbb{Z}} = \{ \dots,-3,-2,-1,0,1,2,3,4, \dots\}.\) - The collection of the positive
**integers**is usually denoted by \(\mathbb{Z_+}\) and we write \({\mathbb{Z_+}} = \{ 1,2,3,4, \dots\}.\) - The collection of the negative
**integers**is usually denoted by \(\mathbb{Z_-}\) and we write \({\mathbb{Z_-}} = \{ -1,-2,-3,-4, \dots\}.\) - The collection of all
**rational numbers**(fractions) is usually denoted by \(\mathbb{Q}\) and we write \({\mathbb{Q}} = \left\{ \frac{a}{b}: a \mbox{ and }b \mbox{ are integers}, \, b \ne 0 \right\}.\) - The collection of all
**irrational numbers**is denoted by \({\mathbb{Q^c}}\). - The collection of all
**real numbers**is denoted by \(\mathbb{R}\). This set contains all of the rational numbers and all of the irrational numbers.

We shall assume the use of the usual addition, subtraction, multiplication, and division as operations and, inequalities (\(<,>,\leq,\geq)\) and equality (\(=\)), are relations on \(\mathbb{R}\).

- The distributive law: If \(a,b\) and \(c\) are real numbers, then \(a(b+c)=ab+ac\) and \((b+c)a=ba+ca.\)
- The commutative law: If \(a\) and \(b\) are real numbers, then \(ab=ba\) and \(a+b=b+a.\)
- The associative law: If \(a,b\) and \(c\) are real numbers, then \(a+(b+c)=(a+b)+c\) and \(a(bc)=(ab)c.\)
- The existence of \(0\): The real number \(0\) exists so that, for any real number \(a, a+0=0+a=a.\)
- The existence of \(1\): The real number \(1\) exists so that, for any real number \(a, a \cdot 1=1 \cdot a=a.\)
- Subtraction: For each real number \(a,\) there exists a real number \(-a,\) so that \(a+(-a)=0=(-a)+a.\)
- Division: For each nonzero real number \(a,\) there exists a real number \(\displaystyle\frac{1}{a},\) so that \(a\left(\frac{1}{a}\right)=\left(\frac{1}{a}\right)a=1.\)

The laws above form the foundation of arithmetic and algebra of real numbers. They are the laws that we have accepted and used with no reserve. They are mentioned here to encourage the reader to develop an appreciation for them and an awareness that they must be respected in all calculations involving real numbers.

Recall that, if \(a\) and \(b\) are real numbers, or \(a, \, b \in \mathbb{R}\) as written in mathematical language, then

- \(a<b\) means that \(a\) is less than \(b.\)
- \(a>b\) means that \(a\) is greater than \(b.\)

**Definitions**

- A real number is called positive if it is greater than \(0\).
- A real number is called non-negative if it is greater than or equal to \(0\).
- An integer \(n\) is an even number if there is an integer \(m\) such that \(n=2m\).
- An integer \(n\) is an odd number if there is an integer \(m\) such that \(n=2m+1\).
- An integer \(a\) is said to be divisible by an integer \(b\) if there is an integer \(m\) such that \(a=bm\). In this case, we can say that \(b\) divides \(a\) and denoted \(b|a\). Further, \(b\) is called a divisor (factor) of \(a\).
- A positive integer \(p\) is called prime if \(p>1\) and the only positive divisors of \(p\) are \(1\) and \(p\).
- A positive integer \(n\) is called composite if there is a positive integer \(m\) such that \(1<m< n\) and \(m|n\).

Note that \(1\) is neither prime nor composite.

The following are axioms for inequalities:

- Trichotomy Law: if \( x \) and\( y \) are real numbers then one and only one of the three statements \(x < y, x = y\) and \(y < x\) is true.
- Transitivity: if \(x, y \) and \(z\) are real numbers and if \( x < y\) and \(y < z\) then \(x < z,\)
- if \(x, y \) and \(z\) are real numbers and if \( x < y\) then \(x + z < y + z,\)
- if \( x \) and \( y \) are real numbers which satisfy \(0 < x \) and \( 0 < y\) then \( 0 < xy,\)

Definition

A theorem is a declarative statement about mathematics for which there is a proof.