A prime number is any number that can only be evenly divided by itself and 1. So for example 2, 3 and 5 are prime, but 6 and 9 are not.

Now, to find all the prime numbers that are factors in a number, we need to first fcheck if the number is even. If it is , then 2 is a factor, and then we keep dividing until the number is odd.

Then, starting at 3, and counting up in 2′s, we try and find a number which divides into the number, each time dividing by the divisor if a match is found.

using System.Collections; public static int[] PrimeFactors(int num) { ArrayList factors = new ArrayList(); bool alreadyCounted = false; while (num % 2 == 0) { if (alreadyCounted == false) { factors.Add(2); alreadyCounted = true; } num = num / 2; } int divisor = 3; alreadyCounted = false; while (divisor <= num) { if (num % divisor == 0) { if (alreadyCounted == false) { factors.Add(divisor); alreadyCounted = true; } num = num / divisor; } else { alreadyCounted = false; divisor += 2; } } int[] returnFactors = (int[])factors.ToArray(typeof(int)); return returnFactors; }

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