In mathematics, the sieve of Eratosthenes, is a simple, ancient algorithm for finding all prime numbers up to any given limit. It does so by iteratively marking as composite (i.e. not prime) the multiples of each prime, starting with the multiples of 2.

Algorithm description
A prime number is a natural number which has exactly Read More →

RSA is a cryptosystem, which is known as one of the first practicable public-key cryptosystems and is widely used for secure data transmission. In such a cryptosystem, the encryption key is public and differs from the decryption key which is kept secret.
In RSA, this asymmetry is based on the practical difficulty of factoring the product Read More →

A permutation, also called an “arrangement number” or “order,” is a rearrangement of the elements of an ordered list S into a one-to-one correspondence with S itself. A string of length n has n! permutation.
Source: Mathword(http://mathworld.wolfram.com/Permutation.html)
Below are the permutations of string ABC.
ABC, ACB, BAC, BCA, CAB, CBA
Here is a solution using backtracking.

These permute2 values themselves Read More →

Someone posed an interesting probelm over at sitepoint the other day.
Given an array of words how can you work out each possible combination?
There are a few methods but here's the PHP code for the cleanest:

\$words = array('red', 'blue', 'green');
\$num = count(\$words);

//The total number of possible combinations
\$total = pow(2, \$num);