"Sift the Two's and Sift the Three's, The Sieve of Eratosthenes. When the multiples sublime, The numbers that remain are Prime." ...anonymous
Sieve of Eratosthenes is a simple but ingenious ancient algorithm for finding all prime numbers up to n.
given a limit n, u've to find all the primes up to n. The built-in prime function of matlab is restricted.