# What is 3 times 7 10

## Get to know prime numbers

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In these explanations you will learn what prime numbers are.

### Prime numbers

A natural number greater than 1 is a prime number if it is only divisible by itself and by 1. This means that a natural number is a prime number if it has exactly two factors. The Greek mathematician Euclid (around 340 BC to 270 BC) proved that there are an infinite number of prime numbers.
Prime number: A natural number is called a prime number if it has exactly two different factors: It is divisible by 1 and by itself.
The prime numbers up to 100 You should know the prime numbers up to 100 by heart.

### Interesting facts about prime numbers

Some prime numbers have special properties. Prime twins If two prime numbers are 2 apart, they are called prime twins. To this day, it is not known whether there are infinitely many pairs of prime twins.
You can see examples of this in the prime number table:
• 3 and 5
• 5 and 7
• 11 and 13
• 17 and 19
Sum of two prime numbers The sum of two prime numbers (without the 2) is always even as the sum of odd numbers. Conversely, it seems that every even number greater than 2 is the sum of two prime numbers.
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The double of a natural number There is at least one prime number between every natural number greater than 1 and its double. Prime numbers as the sum of square numbers Exactly the prime numbers that have the remainder 1 when divided by 4 can be written as the sum of two square numbers.
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