A prime number is one that is divided only by one and by itself.

It is classified as a cousin if it is greater than one and is divisible only by one and by itself. Only natural numbers are classified as prime. Before knowing more about the prime number, it is important to remember some rules of divisibility, which help in the identification of which numbers are not prime.

Divisibility by 2: any even number is divisible by 2. Even numbers are those ending in 0, 2, 4, 6 and 8.

Divisibility by 3: a number is divisible by 3 if the sum of its numbers gives a number divisible by 3.

Divisibility by 4: a number is divisible by 4 if it is divisible twice by 2 or, if its last two digits are divisible by 4.

Divisibility by 5: any number ending in 0 or 5 is divisible by five.

Divisibility by 6: if a number is even and divisible by 3, it will be divisible by 6.

Divisibility by 7: a number is divisible by 7 if the difference between double the last digit and the remainder of the number results in a multiple of 7.

These are the main rules of divisibility. To find each prime number less than 100, we use the “Sieve of Eratosthenes”.