Difference between revisions of "Real number"

Latest revision as of 22:22, 5 January 2022

A real number is a number that falls on the real number line. It can have any value. Some examples of real numbers are: $1, 2, -23.25, 0, \frac{\pi}{\phi}$ , and so on. Numbers that are not real are $\ 3i$ , $\ 3+2.5i$ , $\ 3+2i+2j+k$ , i.e. complex numbers, and quaternions.

The set of real numbers, denoted by $\mathbb{R}$ , is a subset of complex numbers( $\mathbb{C}$ ). Commonly used subsets of the real numbers are the rational numbers ( $\mathbb{Q}$ ), integers ( $\mathbb{Z}$ ), natural numbers ( $\mathbb{N}$ ) and irrational numbers (sometimes, but not universally, denoted $\mathbb{J}$ ). In addition $\mathbb{Z}^{+}$ means positive integers and $\mathbb{Z}^{-}$ means negative integers. The real numbers can also be divided between the algebraic numbers and transcendental numbers, although these two classes are best understood as subsets of the complex numbers.

The set $\mathbb{R}~$

The set of Real numbers is a complete, ordered field under addition and multiplication.

Dedekind developed a method to construct $\mathbb{R}$ , the set of Real numbers from the set of rational numbers, using the very elegant idea of cuts.

Cuts

Let $\mathbb{Q}$ be the set of rational numbers.

Let $\alpha\subset\mathbb{Q}$ be non-empty

We say that $\alpha$ is a cut if and only if

(1) $\alpha\neq\mathbb{Q}$ and $\alpha$ is bounded above

(2) If $a\in\alpha$ , $p\in\alpha\forall p<a$

(3) If $p$ is a rational and $p\in\alpha$ then there exists a rational $q>p$ such that $q\in\alpha$

We define $\mathbb{R}$ to be the set of all cuts $\alpha$

Using this definition, we can show that $\mathbb{R}$ possesess all the properties mentioned above.

Field Axioms

Order Relation

Let $\alpha$ , $\beta$ be cuts

We say that $\alpha<\beta$ iff $\alpha\subset\beta$

Completeness

Let $A\subset\mathbb{R}$

Let $M\in\mathbb{R}$ be an upper bound of $A$

Let $S$ be the set of all rationals $q$ such that $q\notin\alpha$ $\forall$ $\alpha\in A$ .

As $A$ is bounded above, $S$ is non empty.

Consider the set $S'=\{-q|q\in S\}$

We see that $S'$ is a cut, say $\gamma$

Thus, the cut $-\gamma$ is the supremum of set $A$

QED

@@ Line 3: / Line 3: @@
 The set of real numbers, denoted by <math>\mathbb{R}</math>, is a subset of [[complex number]]s(<math>\mathbb{C}</math>). Commonly used subsets of the real numbers are the [[rational number]]s (<math>\mathbb{Q}</math>), [[integer]]s (<math>\mathbb{Z}</math>), [[natural number]]s (<math>\mathbb{N}</math>) and [[irrational number]]s (sometimes, but not universally, denoted <math>\mathbb{J}</math>). In addition <math>\mathbb{Z}^{+}</math> means positive integers and <math>\mathbb{Z}^{-}</math> means negative integers. The real numbers can also be divided between the [[algebraic number]]s and [[transcendental number]]s, although these two classes  are best understood as subsets of the [[complex number]]s.
+==The set <math>\mathbb{R}~</math>==
+The set of Real numbers is a [[Least upper bound|complete]], [[Order relation|ordered]] [[field]] under addition and multiplication.
-== See Also ==
+[[Dedekind]] developed a method to construct <math>\mathbb{R}</math>, the set of Real numbers from the set of [[rational number]]s, using the very elegant idea of '''cuts'''.
+===Cuts===
+Let <math>\mathbb{Q}</math> be the set of rational numbers.
+Let <math>\alpha\subset\mathbb{Q}</math> be non-empty
+We say that <math>\alpha</math> is a '''cut''' if and only if
+(1) <math>\alpha\neq\mathbb{Q}</math> and <math>\alpha</math> is [[bounded]] above
+(2) If <math>a\in\alpha</math>, <math>p\in\alpha\forall p<a</math>
+(3) If <math>p</math> is a rational and <math>p\in\alpha</math> then there exists a rational <math>q>p</math> such that <math>q\in\alpha</math>
+We define <math>\mathbb{R}</math> to be the set of all cuts <math>\alpha</math>
+Using this definition, we can show that <math>\mathbb{R}</math> possesess all the properties mentioned above.
+===Field Axioms===
+===Order Relation===
+Let <math>\alpha</math>, <math>\beta</math> be cuts
+We say that <math>\alpha<\beta</math> iff <math>\alpha\subset\beta</math>
+===Completeness===
+Let <math>A\subset\mathbb{R}</math>
+Let <math>M\in\mathbb{R}</math> be an [[bounded|upper bound]] of <math>A</math>
+Let <math>S</math> be the set of all rationals <math>q</math> such that <math>q\notin\alpha</math> <math>\forall</math> <math>\alpha\in A</math>.
+As <math>A</math> is bounded above, <math>S</math> is non empty.
+Consider the set <math>S'=\{-q|q\in S\}</math>
+We see that <math>S'</math> is a cut, say <math>\gamma</math>
+Thus, the cut <math>-\gamma</math> is the supremum of set <math>A</math>
+<p align="right">QED</p>
+==See Also==
+*[[Field]]
 *[[Natural number]]
-*[[Integer]]
 *[[Rational number]]
 *[[Irrational number]]
@@ Line 14: / Line 61: @@
 [[Category:Definition]]
-[[Category:Number theory]]
+[[Category:Analysis]]

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