## The Sieve of Eratosthenes 1 to 100: An Introduction

The Sieve of Eratosthenes is used to identify prime numbers and composite numbers. We will discuss in detail the topic and find the prime numbers from 1 to 100. By the sieve of Eratosthenes, we have 25 prime numbers and 75 composite numbers between 1 to 100. Eratosthenes sieve method is the easiest way to find prime numbers from given many numbers.

There are 25 numbers between 1 to 100: 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89 and 97

## Eratosthenes Sieve Method

This method is used to identify the prime numbers from a group of natural numbers. In this method, first, we will identify all composite numbers. The remaining numbers are the prime numbers.

## What is the Sieve of Eratosthenes?

Sieve of Eratosthenes is a method by which we can find prime numbers and composite numbers that are less than 10 million.

Sieve of Eratosthenes is also said to be an algorithm because it follows a set of operations.

## Prime Numbers and Composite Numbers

Prime Numbers are those numbers that have only two factors 1 and themselves.

For e.g. 7 has only two factors 1 and 7 itself

Composite Numbers are those numbers that have more than two factors with 1 and themselves.

For e.g. 6 has more than two factors which are 1,2, 3, and 6.

## Sieve of Eratosthenes Prime Numbers 1 - 100

Now we will learn how to find the first 25 prime numbers or prime numbers between 1 to 100 by Sieve of Eratosthenes. We write the number from 1 to 100 like this and follow the given steps:

Step 1: First we write all the natural numbers row-wise and column-wise like the given table.

Step 2: Cross the number 1 as it is not a prime or composite number.

Step 3: Now leave 2 and cross the multiples of 2 as all are composite numbers.

Step 4: Next leave 3 and cross the multiples of 3 as all are composite numbers.

Step 5: Again we will leave 5 and cross the multiple of 5 apart from 5 all are composite numbers.

Step 6: Now leave 7 and cross all multiples of 7.

At this step, we covered all number composite numbers. The rest numbers are prime.

The multiples of 2 from 1 to 100are 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100.

The multiple of 3 from 1 to 100 are 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 84, 87, 90, 93, 96, 99.

The multiple of 4 from 1 to 100 are 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68, 72, 76, 80, 84, 88, 92, 96, 100.

The multiples of 5 are 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100.

The multiple of 6 from 1 to 100 are 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 78, 84, 90, 96.

The multiples of 7 from 1 to 100 are 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98.

The multiples of 8 are also multiples of 2 and 4.

The multiples of 9 are also multiples of 3.

The multiples of 10 are also multiples of 5.

The multiples of 11 are also multiples of 2,3,4,5,6,8,9.

Similarly, the multiples of 12, 13, 14 …, and 99 are marked when we marked the multiples of 2,3,4,5,6,7,8,9,10.

Therefore, we will mark all multiples of the numbers of 2 to the square root of 100 that is 10 and the rest will be prime numbers.

## Prime Numbers Using Sieve of Eratosthenes

After finishing the process we will get all prime numbers between 1 to 100, they are

2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89, and 97. There are 25 prime numbers between 1 to 100.

So this is the way to find prime number sieve by the Sieve of Eratosthenes.

## Interesting Facts

2 is the smallest and only prime number that is even.

Every prime number except 2 is an odd number.

Developer of Sieve Eratosthenes method Greek Mathematician Eratosthenes is also known as the Father of Geography.

## Solved Examples

Q1. Find 99 is a prime or composite number.

Solution. Factors of 99 are 1,3,9 and 11. It has more than two factors except 1 and 99. Therefore, it’s a Composite number.

Q2. Is 37 is a prime number.

Solution: Factors of 37 are 1 and 37, therefore it’s a prime number because it has only two factors 1 and itself.

Q3. Write all Prime numbers between 1 to 50 by the Sieve Eratosthenes method.

Solution: First we prepare the table to find the prime numbers between 1 to 50 and follow the following steps:

First, we write all the natural numbers row-wise and column-wise like the given table.

Cross the number 1 as it is not a prime nor composite number.

Now color the 2 and cross the multiples of 2 as all are composite numbers.

Next color the 3 and cross the multiples of 3 as all are composite numbers.

Again, we will color the 5 number and cross the multiple of 5 apart from 5 all are composite numbers.

Now color the number 7 and cross all multiples of them.

Continue the process till all numbers get color and cross.

Hence, there are 15 prime numbers between 1 to 50, they are 2,3,5,7,11,13,17,19,23,29,31,37,41,43 and 47.

## Key Features

The Prime numbers from 1 to 100 are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, and 97.

Number of composite numbers is 75.

The number of prime numbers from 1 to 100 is 25.

## Practice Problem

1. Find the greatest prime number less than 120.

Answer: 113.

2. Find the sum of the two greatest prime numbers less than 110.

Answer: 226.

## FAQs on The Sieve of Eratosthenes 1 to 100

1. Why is 1 neither prime nor composite?

According to the definition of prime number: numbers that have exactly two factors are known as prime numbers.

1 is divisible by 1. Thus 1 has only one factor.

**Composite Numbers:** A number that has more than 2 factors is known as a composite number.

But 1 has only one factor.

Therefore 1 is neither prime nor composite.

2. Why is the Sieve of Eratosthenes known as an algorithm?

Algorithm is a set of rules or operations that are repeated until we reach the required result.

Sieve of Eratosthenes is a method. In this method, we cross all multiples of 2. Then again we cross all multiples of 3. This process is going all until we identify all composite numbers.

Since Sieve of Eratosthenes consists of a set of operations. That’s why Sieve of Eratosthenes is known as an algorithm.

3. What is the limit of the Sieve of Eratosthenes to find prime numbers?

Sieve Eratosthenes's method is used to find prime numbers less than n where n must be less than 10 million. The limit of the Sieve of Eratosthenes is 10 million.