ruby
def is_prime(n)
    return false if n <= 1
    return true if n <= 3
    return false if ((n % 2) == 0) || ((n % 3) == 0)        
    i = 5
    # check division on 6k+i polinom
    # cause all primes are of the form 6k ± 1, with the exception of 2 and 3.
    while i*i <= n
        return false if n % i == 0 || n % (i + 2) == 0            
        i += 6
    end
    return true
end