An algorithm for making tables of primes. Sequentially write down the integers from 2 to the highest number 
you wish to include in the table. Cross out all numbers 
which are divisible by 2 (every second number). Find the smallest remaining number 
. It is 3. So cross out all numbers 
which are divisible by 3 (every third number). Find the smallest remaining number 
. It is 5. So cross out all numbers 
which are divisible by 5 (every fifth number).
you wish to include in the table. Cross out all numbers which are divisible by 2 (every second number). Find the smallest remaining number . It is 3. So cross out all numbers which are divisible by 3 (every third number). Find the smallest remaining number . It is 5. So cross out all numbers which are divisible by 5 (every fifth number).
Continue until you have crossed out all numbers divisible by 
, where 
is the floor function. The numbers remaining are prime. This procedure is illustrated in the above diagram which sieves up to 50, and therefore crosses out composite numbers up to 
. If the procedure is then continued up to 
, then the number of cross-outs gives the number of distinct prime factors of each number.
, where is the floor function. The numbers remaining are prime. This procedure is illustrated in the above diagram which sieves up to 50, and therefore crosses out composite numbers up to . If the procedure is then continued up to , then the number of cross-outs gives the number of distinct prime factors of each number.
The sieve of Eratosthenes can be used to compute the prime counting function as
 |
which is essentially an application of the inclusion-exclusion principle.
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