How to create the most compact mapping n → isprime(n) up to a limit N?

Question

Naturally, for bool isprime(number) there would be a data structure I could query.
I define the best algorithm, to be the algorithm that pr

Answer 1

Here's my take on the answer:

def isprime(num):
    return num <= 3 or (num + 1) % 6 == 0 or (num - 1) % 6 == 0

The function will return True if any of the properties below are True. Those properties mathematically define what a prime is.

1. The number is less than or equal to 3
2. The number + 1 is divisible by 6
3. The number - 1 is divisible by 6