Natural number

Natural numbers, also called counting numbers, are the numbers used for counting things. Natural numbers are the numbers small children learn about when they first started to count. Natural numbers are always whole numbers (integers excluding negative numbers) and often exclude zero, in which case one is the smallest natural number. The set of natural numbers can be represented by the symbol .

There is no largest natural number. The next natural number can be found by adding 1 to the current natural number, producing numbers that go on "forever". There is no natural number that is infinite in size. Any natural number can be reached by adding 1 enough times to the smallest natural number.

Non-natural numbers
The following types of numbers are not natural numbers:
 Numbers less than 0 (negative numbers), for example, −2 and −1
 Fractions, for example, ½ and 3¼
 Decimals, for example, 7.675
 Irrational numbers, for example,  and   (pi)
 Imaginary numbers, for example,  (i)
 infinity, for example,  and 
0 (if not include as a natural number)

Basic operations

 Addition; The sum of two natural numbers is a natural number. 
 
 
 
 Multiplication: The product of two natural numbers is a natural number. 
 
     
 
 
  

 Ordering: Of two natural numbers, if they are not the same, then one is bigger than the other, and the other is smaller.  m = n or m > n or m < n
 if l > m then l + n > m + n
 if l > m and l > 0 then l x n > l x m
 Zero is the smallest natural number: 0 = n or 0 < n
 There is no largest natural number n < n + 1

 "Subtraction": If n is smaller than m then m minus n is a natural number. If n < m then m - n = p. 
 if l - m = n then l = n + m
 if n is greater than m, then m minus n is not a natural number
 if l = m - n and p < n then l > m - p

 Division: If  then 

 Mathematical induction: If these two things are true of any property P of natural numbers, then P is true of every natural number
 if P is true of 1
and if P of n then P of n+1
then P is true of all natural numbers

Special natural numbers

Even numbers: If n = m x 2, then n is an even number
 The even numbers are 0, 2, 4, 6, and so on. Zero is the smallest (or first) even number.
Odd numbers: If n = m x 2 +1, then n is an odd number
 A number is either even or odd but not both.
 The odd numbers are 1, 3, 5, 7, and so on.
Composite numbers: If n = m x l, and m and l are not 0 or 1, then n is a composite number.
 The composite numbers are 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 21 and so on.
Prime numbers: If a number is not 0, 1, and not a composite number, then it is a prime number.
 The prime numbers are 2, 3, 5, 7, 11, 13, 17 and so on. Two is the smallest (or first) prime number. Two is the only even prime number.
 There is no biggest prime number. 
Square numbers: If n = m x m, then n is a square. n is the square of m.
 The squares are 0, 1, 4, 9, 16, 25, 36, 49 and so on.

How to write it 
 or  is the way to write the set of all natural numbers. Because some people say 0 is a natural number, and some people say it is not, people use the following symbols to talk about the natural numbers:

Related pages
 1 (number)
 Integer
 Zero
Number theory

References