# POW 1UP

March 27, 2010 5:19 AM Subscribe

Is there a closed-form method for finding the unique subsets from the power set of prime factors of an integer?

Let's say I start with the integer 28, with prime factors {2, 2, 7}.

If I construct a power set from this set of factors, I get:

{{nil}, {2}, {2}, {7}, {2,2}, {2,7}, {2, 7}, {2, 2, 7}}

I can reduce this to a set of divisors by multiplying the elements inside the non-empty subsets, to get {2, 7, 4, 14, 28} (adding {1} as a default divisor).

However, two of the subsets are duplicates: {2} and {2,7}, so I am doing unnecessary calculations by simply iterating through all subsets.

I could loop through the subsets and keep a hash table of multiplied elements-as-keys or some similar brute force approach. But as a power set grows quickly, there could be a very large number of subsets.

Is there a smarter, more general way to do this, if I know what the labels (prime factors) are, something like a unique permutation of the factors, where some factors might be repeated?

(Note: This isn't homework.)

Let's say I start with the integer 28, with prime factors {2, 2, 7}.

If I construct a power set from this set of factors, I get:

{{nil}, {2}, {2}, {7}, {2,2}, {2,7}, {2, 7}, {2, 2, 7}}

I can reduce this to a set of divisors by multiplying the elements inside the non-empty subsets, to get {2, 7, 4, 14, 28} (adding {1} as a default divisor).

However, two of the subsets are duplicates: {2} and {2,7}, so I am doing unnecessary calculations by simply iterating through all subsets.

I could loop through the subsets and keep a hash table of multiplied elements-as-keys or some similar brute force approach. But as a power set grows quickly, there could be a very large number of subsets.

Is there a smarter, more general way to do this, if I know what the labels (prime factors) are, something like a unique permutation of the factors, where some factors might be repeated?

(Note: This isn't homework.)

*two of the subsets are duplicates: {2} and {2,7}, so I am doing unnecessary calculations by simply iterating through all subsets.*

If you are actually representing these using some kind of set data structure, duplicates should be removed for you automatically. By definition sets don't contain duplicates (e.g. {{0}, {0}} is the same set as {{0}}). However, if you are using a multiset, it won't do this for you. If you are rolling your own data structure, maybe the thing to do would be to see how a standard implementation of a set would do this. (Or just use one.)

posted by advil at 6:20 AM on March 27, 2010 [2 favorites]

Exactly what madcaptenor said: write out all the unique prime factors and how often each one occurs. Then, every element of the factor power set can be written in terms of taking f_1 of the first factor, f_2 of the 2nd factor, etc...

On that note, if there are factors p_1, p_2, ... p_m with counts e_1, e_2, ... , e_m, the number has (1+e_1) * (1+e_2) ... * (1+e_m) factors including 1 and itself.

posted by bsdfish at 8:07 AM on March 27, 2010

On that note, if there are factors p_1, p_2, ... p_m with counts e_1, e_2, ... , e_m, the number has (1+e_1) * (1+e_2) ... * (1+e_m) factors including 1 and itself.

posted by bsdfish at 8:07 AM on March 27, 2010

*(Note: This isn't homework.)*

I'm pretty sure it's Euly, though.

Anyway, I agree with the above: Keep track of unique primes and the powers thereof.

posted by DU at 8:43 AM on March 27, 2010

*If you are actually representing these using some kind of set data structure, duplicates should be removed for you automatically.*

That would be language or implementation dependent, and a kind of a cheat. My hope was to find a method for generating a power set that automatically removes duplicates, without relying on the vagaries of a particular language. But instead of using a power set, it seems better to just use an ordered tuple approach, once I have the factorization.

posted by Blazecock Pileon at 4:48 PM on March 27, 2010

*Then you want p_1^(f_1) ... p_m^(f_m) for each m-tuple (f_1, ..., f_m) where f_k is between 0 and e_k (inclusive).*

Isn't that also just the list of factors?

posted by albrecht at 7:38 PM on March 27, 2010

albrecht, I thought the list of factors is what Blazecock was asking for.

posted by madcaptenor at 6:53 AM on March 28, 2010

posted by madcaptenor at 6:53 AM on March 28, 2010

Oh, sorry; I misunderstood the question. In that case, your answer is definitely the way to go.

posted by albrecht at 7:35 AM on March 28, 2010

posted by albrecht at 7:35 AM on March 28, 2010

No, I have the factors. I was trying to build a count of divisors. Using a power set will introduce duplicates. So I just need to generate permutations of factors a different way.

posted by Blazecock Pileon at 3:25 AM on March 30, 2010

posted by Blazecock Pileon at 3:25 AM on March 30, 2010

This thread is closed to new comments.

Then you want p_1^(f_1) ... p_m^(f_m) for each m-tuple (f_1, ..., f_m) where f_k is between 0 and e_k (inclusive).

posted by madcaptenor at 5:25 AM on March 27, 2010 [3 favorites]