A prime number is a number that can only be divided by 1 and by itself.
Prime numbers from 1 to 1000
There are 168 prime numbers between 1 and 1000. See in the table:
How do you know if a number is prime?
There are a few methods to find out if a number is prime. Let's see some:
Divide by other prime numbers
One of the techniques to identify if a number is prime is to divide it by other prime numbers, always in order (2,3,5,7,11...). In this method there are two possible results:
- If the result of the division is less than the divisor, the number is prime.
- If the result of the division has a remainder 0, the number is not prime.
Keep doing the division until you find the number where the result of the division is less than the prime divisor number. For a better understanding, look at this example with the number 19.
- Divide by 2 - 19 divided by 2 equals 9 and 1 is left.
- Divide by 3 - 19 divided by 3 equals 6 and 1 is left.
- Divide by 5 - 19 divided by 5 equals 3 and 4 is left.
- Divide by 7 - 19 divided by 7 equals 2 and you have 5.
Since the result of dividing 19 by 7 is less than the divisor number (2 is less than 7), we already know that 19 is a prime number.
If the result of the division has a remainder of 0, the number is not prime. See the example with number 18.
- Divide by 2 -18 divided by 2 equals 9 and 0 is left.
Since the result of the division has a remainder of 0, we know that 18 is not a prime number.
Eratosthenes Sieve Method
The method, created by the mathematician Eratosthenes, allows you to find prime numbers by building a table. Eratosthenes was a mathematician who lived in Ancient Greece and, in addition to creating the method, he is also known to be responsible for calculating the Earth's circumference.
To understand how it works, follow these steps:
Make a table with the numbers from 1 to 100 (or from 1 to 1000, as needed).
Cross out all numbers in the table that are multiples of 2 (4, 6, 8, 10, 12...). Attention: the number 2 must not be crossed out as it is a prime number.
Cross out all numbers that are multiples of 3 (6, 9, 12, 15, 18...). The number 3 should not be crossed out because it is also a prime number.
Cross out all numbers multiple of 5 (10, 15, 20, 25, 30, 35...). The number 5 should not be crossed out because it is a prime number.
Eliminate all numbers that are multiples of 7 (17, 21, 28, 35, 42, 49...). 7 is a prime number and should not be crossed out.
Uncrossed numbers are prime numbers..
Application of the Sieve of Eratosthenes to identify prime numbers between 1 and 100.
Curiosity
All prime numbers are odd, there is only one exception: the number 2 is the only prime number that is even.
Also know the Roman numbers and see the meanings of number and MMC and MDC.
