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 Testing for Primality, a selection of answers from the Dr. Math archives. Prime Numbers - Dr. Math FAQ What is a prime number? How can you find prime numbers? What's the 'Sieve of Eratosthenes'? What's the largest known prime? From the archives: How does the Sieve of Eratosthenes Work? How does the Sieve of Eratosthenes work for finding prime numbers? Testing Prime Numbers Besides the Sieve of Eratosthenes, what other methods can be used to determine all prime numbers within a given range? Is there a more efficient method? Large Prime Numbers Is there an algorithm to determine whether a very large number is prime? Primality Testing Is there any fomula to find if a number is a prime? Prime Number Tests Is the number 55409243 prime? How can you test to see whether a number is prime? Primality Test I want to write a program using Pascal that will verify whether a number is prime. How Many Primes Are Known? How many prime numbers are currently known? Largest Prime Number What is the largest prime number yet found? No Largest Prime Number What's the largest prime number? (proof by contradiction). Testing primality of 32-bit numbers What is the best (fastest) way to test if an arbitrary 32-bit number is prime? Integer Root Checking Is there a quick way to check whether a number has any roots that are whole numbers?