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# I cant understand prime numbers

I don't understand how to work out what a prime number is

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A prime number is a positive whole number (natural number) which cannot be divided into smaller whole numbers. In short, it can only be divided (integer division) by itself and 1.
First prime number is 2, then 3, 5, 7, 11, 13, 17, 19,
To generate a list of prime numbers the Greek Eratoshenes used a method called now the sieve Eratosthenes.

1. Write all the natural numbers up to some chosen limit (100, 259, any limit )
2. Remove 1 by crossing it ( 1 is no longer considered prime).
3. The first prime number is 2. Circle the number 2.
4. Go through the whole list crossing out all the numbers that are multiple of 2, that is 4, 6, 8,10,12, ...
5. Repeat.
6. The next prime number is 3 since it was not crossed out as a multiple of 2. Circle 3.
7. Go through the whole list crossing all the numbers that are multiples of 3, and which have not already been crossed out as multiples of 2 .
8. Number 4 has been crossed out already.
9. Next prime number is 5. Circle 5
10. Go through the whole list crossing all the numbers that are multiple of 5 and which have not been crossed out already as multiples of 2, or 3
11. Repeat:
12. Next prime number is 7. Circle 7
13. Go through the whole list again, crossing out the multiples of 7, and so on.
If you want to test the primality of a positive integer (odd number) starting by dividing it by 3 (use divisibility rules or otherwise), then divide by 5, by seven, and all subsequent prime numbers. If it is not divisible by any prime number, keep going but do not exceed the prime number that is closest yet smaller than square root of your number.

Enjoy.

Posted on Nov 26, 2013

• k24674 Nov 26, 2013

OOPs lines 7 and 8: a method called now the sieve of Eratosthenes.

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## Related Questions:

### Examples of prime numbers?

Presume you know that prime number is divisible only by itself. See here:

http://primes.utm.edu/lists/small/1000.txt

Oct 13, 2014 | Computers & Internet

### What is a prime number ?

A prime number is a whole number greater than one which has no divisors except one and itself. A composite number is a whole number greater than one that isn't prime, that is, a product of two or more whole numbers. See http://en.wikipedia.org/wiki/Prime_number

Jul 07, 2014 | Computers & Internet

### What criteria make a number a prime number?

It is any number only divisible by itself and 1 (giving whole number results). A list here of the first 1000

http://primes.utm.edu/lists/small/1000.txt

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### Whats prime numbers

Prime numbers are those numbers (greater than 1) that cannot be divided by any number except themselves and one.
The Greek Eratosthenes created a method to find out these prime numbers, although it only worked over a limited range:
1) Write out the numbers from 1 to 100 in ten rows of 10.

2) Cross off number 1, because all primes are greater than 1.
3) Number 2 is a prime, so we can keep it, but we need to cross off the multiples of 2 (i.e. even numbers).
4) Number 3 is also a prime, so again we keep it and cross off the multiples of 3.
5) The next number left is 5 (because four has been crossed off), so we keep it and cross of the multiples of this number.
6) The final number left in the first row is number 7, so cross off its multiples.
7) You have finished. All of the "surviving" numbers (coloured in white below) on your grid are prime numbers.

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### What is the prime number between 63 and 73??

I beleive its-- 67 and 71, good-day! prime numbers are a number that cant be divided equally.

Aug 25, 2011 | Home

### How to explain that all numbers ending with 1,3,5,7,9 are prime numbers.in other words can I say that all odd numbers are prime numbers?

No not all odd numbers are prime numbers. A prime number is one that cannot be divided equally by any other number then itself or 1. Therefore numbers such as 9 is not a prime number because it can also be evenly divided by 3Hope this answers your question. Thank you.

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### What is prime factorization?

"Prime Factorization" is finding which prime numbers multiply together to make the original number.

Example : What are the prime factors of 12 ? It is best to start working from the smallest prime number, which is 2, so let's check:

12 ÷ 2 = 6
Yes, it divided evenly by 2. We have taken the first step!
But 6 is not a prime number, so we need to go further. Let's try 2 again:

6 ÷ 2 = 3
Yes, that worked also. And 3 is a prime number, so we have the answer:

12 = 2 × 2 × 3

As you can see, every factor is a prime number, so the answer must be right.

Note: 12 = 2 × 2 × 3 can also be written using exponents as 12 = 22 × 3

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Hey i found this great new site offering Adobe Photoshop Cs2 serial numbers, here's the link:
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I hope this will help

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### WAP in C language to print all the prime numbers & perfect numbers between 0 to 1000.

I wrote this program for you. You can see it here: pastebin
You should understand how this works and make your own. That way you'll remember it and gain some experience. Not to mention satisfaction.

Apr 10, 2009 | Microsoft Computers & Internet

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