Natural numbers are whole, non-negative numbers that are used for counting objects.
The set of natural numbers is denoted by N.
A prime number is a natural number greater by 1, and which can be divided evenly by 1 or itself.
A composite number is a whole number that can be divided evenly by numbers other than 1 or itself.
The table below shows the prime and composite numbers from 1 to 100.
*Prime numbers – blue squares
*Composite numbers – white squares
*1 is neither a prime nor a composite number.
A twin prime is a pair of prime numbers that differ by two.
Two integers are relative prime or co-prime if the only positive integer that evenly divides both of them is 1 (that is, their greatest common divisor is one).
Let us consider the divisors of the numbers 18 and 23.
Divisors of 18: 1, 2, 3, 6, 9, 18
Divisors of 23: 1, 23
The only common number in these two sets of divisors is 1.
So, 18 and 23 are co-primes.