def get_primes7(n):
    """
    standard optimized sieve algorithm to get a list of prime numbers
    --- this is the function to compare your functions against! ---
    """
    if n < 2:  return []
    if n == 2: return [2]
    # do only odd numbers starting at 3
    s = list(range(3, n+1, 2))
    # n**0.5 simpler than math.sqr(n)
    mroot = n ** 0.5
    half = len(s)
    i = 0
    m = 3
    while m <= mroot:
        if s[i]:
            j = (m*m-3)//2  # int div
            s[j] = 0
            while j < half:
                s[j] = 0
                j += m
        i = i+1
        m = 2*i+3
    return [2]+[x for x in s if x]