What is a Prime Number

Prime number is an integer number greater than 1 that has only two factors: 1 and itself.

For example, 5 is prime number because it has no other factors except 1 and 5.

By convention 1 is not prime, so prime numbers start from 2.

Interesting fact. 2 is the only even prime number. Other even numbers are not prime, because they have factor 2.

So, prime numbers are following: 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,...

Example 1. Determine whether 25 is prime.

Not prime because 25 is divisible by 5, so 5 is factor of 25.

Next example.

Example 2. Determine whether 61 is prime.

It is prime number because it has only two factors: 1 and 61.

Now, practice a bit.

Exercise 1. Determine whether 71 is prime.

Answer: prime.

Next exercise.

Exercise 2. Determine whether 75 is prime.

Answer: not prime.