For 10 points—this describes what "strainer" that identifies prime numbers?
To use it, first note that if one factor of a number other than a perfect square is greater than its square root, then the other factor will be less than its square root. Hence all multiples of primes greater than the square root of n need not be considered. Make a list of all the integers less than or equal to n and greater than one. Strike out the multiples of all primes less than or equal to the square root of n. Then the numbers that are left are the primes. For 10 points—this describes what "strainer" that identifies prime numbers?
To use it, first note that if one factor of a number other than a perfect square is greater than its square root, then the other factor will be less than its square root.
{sieve of Eratosthenes}