**From natural integers **

to real numbers

**• Natural integers
**

The simplest numbers are the so called natural
integers:

**0 , 1 , 2 , 3 , ... **

The set of natural integers is denoted by ** N **. Among these numbers there is an important
subclass whose elements are called prime numbers. The prime numbers are defined as the natural
integers greater than one which are only divisible by one and themselves. The set of prime numbers,
denoted by ** P **, is infinite and its first elements are :
** 2 , 3 , 5 , 7 , 11 , 13
, ... **

Observe that **2 ** is the only even
prime number.
**• Rational integers
**

By considering the opposite of the natural integers,
we obtain the set of rational integers, denoted by ** Z **, whose elements are :

**... , -3 , -2 , -1 , 0
, 1 , 2 , 3 , ... **

**• Rational numbers
**

By taking the inverse of all non-zero rational integers and their multiples,
we obtain the set of rational numbers, denoted by ** Q **. For example
**-135/11 ** is a rational number. Rational numbers are
commonly written in decimal expansion. For example

**1/2 = 0.5
1462/50
= 29.24
230/3 =76.666666... **

and
**-135/11 = -12.27272727....
**

We recall the meaning of this writting
**
1/2 = 5/10
1462/50 = 29 + 2/10 + 4/100
**

and
**230/3 = 76 + 6/10 + 6/100 + 6/1000 + 6/10000
+.... **

or else
**230/3 = 7*10**^{1} + 6*10^{0}
+ 6*10^{-1} + 6*10^{-2} .....

So every positive rational number can be written as
**x=a**_{n}10^{n} + a_{n-1}10^{n-1}
+ a_{n-2}10^{n-2} +......

Here the digits **a**_{i}
are natural integers between 0 and 9. Notice that the multiplicative symbol is traditionally omited between ** a**_{i} and ** 10**^{i} . Besides, clearly the sequence of the digits may be finite
or infinite.

If **n > -1 ** , then the begining of the
expansion **u = a**_{n}10^{n} +
a_{n-1}10^{n-1}+.... + a_{0} is a natural
integer called the integral part of **x, **
while the remaining part ** v = a**_{-1}10^{-1}
+ .... is the fractional part of **x
. **

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