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    <p>Since you already have a list of the prime factors, what you want to do is to compute the powerset of that list. </p> <p>Now, one problem is that you might have duplicates in the list (e.g. the prime factors of 20 = 2 * 2 * 5), but sets don't allow duplicates. So, we can make each element of the list unique by projecting it to a structure of the form {x, y} where x is the prime and y is the index of the prime in the list.</p> <pre><code>var all_primes = primes.Select((x, y) =&gt; new { x, y }).ToList(); </code></pre> <p>Now, <code>all_primes</code> is a list of the form {x, y} where x is the prime and y is the index in the list.</p> <p>Then we compute the power set (definition of <code>GetPowerSet</code> below):</p> <pre><code>var power_set_primes = GetPowerSet(all_primes); </code></pre> <p>Hence, <code>power_set_primes</code> is an <code>IEnumerable&lt;IEnumerable&lt;T&gt;&gt;</code> where <code>T</code> is the anonymous type <code>{x, y}</code> where x and y are of type <code>int</code>.</p> <p>Next, we compute the product of the each element in the power set</p> <pre><code>foreach (var p in power_set_primes) { var factor = p.Select(x =&gt; x.x).Aggregate(1, (x, y) =&gt; x * y); factors.Add(factor); } </code></pre> <p>Putting it all together:</p> <pre><code>var all_primes = primes.Select((x, y) =&gt; new { x, y }).ToList(); //assuming that primes contains duplicates. var power_set_primes = GetPowerSet(all_primes); var factors = new HashSet&lt;int&gt;(); foreach (var p in power_set_primes) { var factor = p.Select(x =&gt; x.x).Aggregate(1, (x, y) =&gt; x * y); factors.Add(factor); } </code></pre> <p>From <a href="http://rosettacode.org/wiki/Power_Set" rel="nofollow noreferrer">http://rosettacode.org/wiki/Power_Set</a> for implementations of powerset.</p> <pre><code>public IEnumerable&lt;IEnumerable&lt;T&gt;&gt; GetPowerSet&lt;T&gt;(List&lt;T&gt; list) { return from m in Enumerable.Range(0, 1 &lt;&lt; list.Count) select from i in Enumerable.Range(0, list.Count) where (m &amp; (1 &lt;&lt; i)) != 0 select list[i]; } </code></pre>
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    1. COI dont follow. Factors is the list of prime factors that I found? And since what I want is all combinations of the factors that multiply to less than or equal to to the number, there has to be a better way.
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    2. COOpps, that shouldn't be Math.Floor... editing...
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    3. COthis does not really work though. currently, if a prime goes in more than itself times, this will not give the correct factors (this happens a lot, i.e. every time 9 is a factor...)
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