def prime3(a):
    if a < 2: return False
    if a == 2 or a == 3: return True # manually test 2 and 3
    if a % 2 == 0 or a % 3 == 0: return False # exclude multiples of 2 and 3

    maxDivisor = a**0.5
    d, i = 5, 2
    while d <= maxDivisor:
        if a % d == 0: return False
        d += i
        i = 6 - i # this modifies 2 into 4 and viceversa

    return True