# Prime Factor Problem Cheat Sheets

`1`

is not a prime number.

A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.

## Finding Prime Numbers

### Sieve of Eratosthenes

A basic implementation of the sieve of Eratoshenes I wrote:

```
def primes(max_value)
search_space = Array.new(max_value, true)
prime_range = (2..max_value)
prime_range.each do |i|
if search_space[i]
j = 2 * i
while j <= max_value
search_space[j] = false
j += i
end
end
end
prime_range.map{ |i| i if search_space[i] }.compact
end
```