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### Browsed byMonth: October 2018

Using Boolean Algebra to Find all Maximal Independent Sets in a Graph

## Using Boolean Algebra to Find all Maximal Independent Sets in a Graph

Graph theory may be one of the most widely applicable topics I’ve seen in mathematics. It’s used in chemistry, coding theory, operations research, electrical and network engineering, and so many other places. The subject is mainly credited to have begun with the famous  Seven Bridges of Königsberg problem posed by Leonard Euler in 1736. Frank Harary should also be credited with his massive work in bringing applications of graph theory to the sciences and engineering with his famous textbook written in 1969.

My own research forced me to stumble into this area once my  research partner, Jason Hathcock, suggested we explore the idea of viewing dependency relations in the sequences of variables we were studying as digraphs. Since then, I’ve been buried in graph theory texts, finding a wealth of fascinating topics to explore.

Of this article’s particular interest is finding all maximally independent sets in a graph using Boolean algebra.

## What’s a maximally independent set?

Firstly, what’s an independent set?

Definition (Independent Set): A set of vertices of a graph is independent if no two vertices in the set are adjacent.

If we take a look at the digraph above (from our paper on vertical dependence), and look at the underlying graph1, $\{1,6,11\}$ form an independent set, as an example. There are lots more, and of varying sizes. Of particular interest here are maximal independent sets.

Definition:(Maximal Independent Set): An independent set to which no other vertex in the graph can be added to retain the independence property

An example from the graph above is $\{2,3,4,5,13\}$. If we added any other vertex to that set, it would be adjacent to some vertex already in there.

A few notes:

(1) There are many maximal independent sets in a graph, and they may not all have the same cardinality.

(2) Maximal and maximum are not the same thing. An independent set may be a maximal independent set without being the largest independent set in the graph. The largest cardinality among all the maximal independent sets is called the independence number of the graph and is denoted $\beta(G)$.

## Why do we care about maximal independent sets?

Of the many applications that arise, one in particular is in coding theory. We want to find the largest error correcting codes we can, particularly in internet transmissions that can lose packets. A paper discussing this can be found here. (Paywall warning). We’ve discussed some basics of coding theory on this site as well. Finding error correcting codes with desirable properties is equivalent to solving the problem of finding maximal independent sets. The purpose of this article isn’t to discuss the applications here, but I’ve learned long ago that no one will keep reading unless I mention at least one application.

## Finding a maximal independent set

Finding a maximal independent set is relatively simple. Start with any vertex $v \in V(G)$. Add another vertex $u$ that is not adjacent to $v$. Continue adding vertices that are not adjacent to any already in the set. For a finite graph2, this process will terminate and the result will be a maximally independent set.

Will it be one of largest cardinality? Not necessarily.

For example, using one more of our dependency graphs generated by $\alpha(n) = \sqrt{n}$, we can take the order to be 24 as shown, and build a maximal independent set starting with vertex 3. Note that none of vertices 9-15 or 1 can be in the set, since they’re all adjacent to vertex 3. Vertex 2 is not adjacent to vertex 3, so we add it into our set: $V = \{2,3\}$. Now, the next vertex we add can’t be adjacent to either 2 or 3, so that rules out 1, 9-15, and 4-8. Grab vertex 16. Now $V = \{2,3,16\}$. Notice that none of the remaining vertices are adjacent to any of the previous vertices. Continuing this process, we’ll get that $V = \{2,3,16,17,18,19,20,21,22,23,24\}$. Notice that if we add any other vertices to this set, they’ll be adjacent to something already in it.

## Finding all Maximal Independent Sets

We’re rarely interested in just finding one maximal independent set. We’d prefer to find them all, and doing it by inspection is not very palatable. The heart of the article is an admittedly not optimal but still interesting way to find all maximal independent sets for reasonably small graphs.

We’ll illustrate the method on the 6-node graph above.

### Getting started

First, we’ll assign a Boolean variable to each vertex according to its inclusion in a maximal independent set. For example $A = 1$ implies $A$ is in the maximal independent set. Recall from Boolean algebra that

$$x+y = \left\{\begin{array}{lr}1, & x = 1 \text{ or } y = 1 \text{ or } (x=y=1)\\0,&x=0 \text{ and } y=0\end{array}\right.$$

Remark: $x+y$ is just another way of writing a union. This isn’t addition mod 2 here.

$$xy=\left\{\begin{array}{lr}1, & x = 1 =y\\0,&\text{ otherwise}\end{array}\right.$$

What we’ve done here is set up inclusion into our maximal independent sets in a Boolean fashion. So $x+y = 1$ corresponds to the inclusion of either vertex $x$ OR vertex $y$ OR both vertices $x$ and $y$. Similarly, $xy = 1$ corresponds to the inclusion of both vertices $x$ and $y$.

Now, we can express an edge of a graph as a Boolean product $xy$, where $x$ and $y$ are the vertices at either end of the edge.

Finally, set up the sum of all edges and call it $\phi$:

$$\phi = \sum xy \text{ for all } (x,y) \in G$$

For our graph above,

$$\phi = AB + AD + AE + BC + CE + CF + DE + EF$$

### Why did we do this?

For a vertex to be in an independent set, it can’t be adjacent to any other vertices in the set. Put another way, for each edge, we can only have at most one of the vertices that make it up. If we include $A$ in the independent set $V$, then $B$ cannot be in there.

Returning to our $\phi$, note that its value under Boolean algebra can only be 0 or 1. If $\phi = 1$, then at least one edge has both of its vertices “on”. This means, only combinations of $A, B, C, D, E, F$ that yield $\phi = 0$ will give us a maximally independent set.

### Solving the problem

Our goal now is to find all combinations of our Boolean vertex variables that yield $\phi = 0$. As it turns out, solving this directly is pretty annoying3. If we want $\phi = 0$, that’s logically equivalent to seeking $\phi^{c} = 1$, where $\phi^{c}$ is the Boolean complement (or negation) of $\phi$

Recall from Boolean algebra the following4:

\begin{aligned}(xy)^{c}&=x^{c}+y^{c}\\(x+y)^{c} &= x^{c}y^{c}\end{aligned}

So, if we take $\phi^{c}$ for our graph above,

\begin{aligned}\phi^{c}&=(A^{c}+B^{c})(A^{c}+D^{c})(A^{c}+E^{c})(B^{c}+C^{c})(C^{c}+E^{c})\\&\quad(C^{c}+F^{c})(D^{c}+E^{c})(E^{c}+F^{c})\end{aligned}

What does the negation here actually mean? By taking the complement, instead of finding vertices to include, now we’re finding vertices to excludeWhen we multiply this expression out, we’ll get a sum of terms, where each term is a product of complements of our original Boolean variables. To get $\phi^{c} = 1$, all we need is one of those terms to be 1. To get a term to be 1, all members of the product must themselves be 1, meaning each term gives us a set of variables to exclude. Excluding these variables gives us one maximally independent set for each term, so this gives us all the maximally independent sets.

The nice thing about dealing with Boolean arithmetic is that we can program a computer to do this for us. Any time we can invoke a relationship with Boolean algebra, we can enlist a friendly helpful computer.

### Finishing the example

We’ll do it by hand here, because I’m old-school like that. For larger graphs, obviously one would want to enlist some computational help, or just be very patient. We’ll remember a few other rules for Boolean algebra before we finish5:

\begin{aligned}xx &=x\\x+x &=x\\x +xy&=x\end{aligned}

After an insane amount of tedious Boolean algebra,

$$\phi^{c} = A^{c}C^{c}E^{c}+A^{c}B^{c}E^{c}F^{c}+A^{c}C^{c}D^{c}F^{c}+B^{c}C^{c}D^{c}E^{c}+B^{c}D^{c}E^{c}F^{c}$$

Recall that each term now tell us which sets of vertices to exclude from a maximal independent set. We negated the question logically. That means we have 5 maximal independent sets:

$$\{B,D,F\}, \{C,D\}, \{B,E\}, \{A,F\}, \{A,C\}$$

We can actually say what the independence number is as well, since we just have to find the maximum cardinality among the sets listed. For this graph, $\beta(G) = 3$.

## Conclusion

I happened to find this interesting, and ended up obsessed with it for a day, much to the chagrin of my daily planner, which expected me to be working on writing my research monograph. I tried several different ways of solving this beyond the one given. I tried using the direct equation $\phi$, and tried using regular arithmetic on just $\{0,1\}$, setting up a sort-of structure function similar to the reliability block diagrams detailed here.

I always hesitate to blame the method, and rather my own arithmetic errors, but I didn’t have much luck with the structure-function method, though I may try again to see if it’s an equivalent method. I believe it should be.

Looking at $\phi^{c}$ makes more sense after playing with this problem for some hours. The sum/union is quite nice, because it neatly separates out the various sets to exclude. It’s a better exploitation of Boolean algebra than trying to work with $\phi$ but aiming for a sum of 0. I still think it should be possible to work with it directly, even if not advisable. If I decide to torture myself with it further, and end up with something to write about, perhaps I’ll append here.

I always end up ending my articles with some takeaway. I don’t have much of one here, except it was a curiosity worth sharing. Perhaps a decent takeaway is to reveal a bit of the tedium and dead-ends mathematicians can run into when exploring something. That’s just part of research and understanding. It’s entirely possible to spend hours, days, weeks on something and all you conclude is that the original method you saw is definitely superior than the one you were trying to develop.

Beyond Cookbook Mathematics, Part 2

## Beyond Cookbook Mathematics, Part 2

The previous article discussed the importance of definitions to mathematical thought. We looked at a definition (of an end-vertex in a graph), and picked it apart by finding multiple ways to look at it. We also directly used the definition in a practical manner to find “weak links” in a network. This time, we’ll look at the structure of mathematical theorems and proofs, and how we can read the proofs to understand the theorems.

A mathematical theorem (informally) is a statement that takes the following structure:

IF {we have this stuff}, THEN {we get to say this about other stuff}.

Let’s pick this apart a bit even at the abstract level, because even this simple structure is important when we consider using theorems:

## IF {we have this stuff}:

That IF is really important. You can’t move to the THEN without the IF part being true. Let’s say a particular theorem (say, the Central Limit theorem), has a set of requirements in the IF part. We have to have all of those parts satisfied before we can invoke the conclusion. I see data scientists just invoking “Central Limit Theorem” like a chant when they are analyzing a dataset. However, in many cases, their data does not fit the IF conditions given. The Central Limit Theorem requires, among other things, that the random variables be independent. No independence, no Central Limit Theorem. Do not move forward.

Many complaints I hear, especially in statistics, revolve around “you can use statistics to say anything you like.” No. No you cannot. It only appears that way because practitioners are applying theorems blindly without ensuring the hypotheses (the IF bits) are all met. A theorem written and proven is not some magic silver bullet that allows for universal application and use.

## IF -> THEN:

When we write (or read) a proof, we assume the IF part is true, and use those conditions in the IF to logically deduce the conclusions in the THEN.

(A remark: I just described a particular method of proof—the direct proof. There are other methods by which we may prove a statement that are logically equivalent to a direct proof such as proof by contradiction, or proof by contrapositive. Since the idea here is to understand how all the parts of a theorem and proof work, we’ll stick with direct proofs here. We’re also discussing a particular type of logical statement—a one directional implication. We can have bidirectional statements as well, but these are an extension of understanding this first type, so we’ll start here.)

The first part discussing definitions is essential to understanding the IF part of our theorem and how this part connects to our “then” conclusion. Suppose we take the statement

If a real-valued function $f$ is differentiable, then $f$ is continuous.

We can use the statement without understanding. As long as we understand the definitions reasonably well (as in, we know how to tell if a function is differentiable), then we say it’s continuous and move on with our day. This isn’t good enough anymore. Why does differentiability imply continuity? A good proof should illuminate this for us. My suggestion at this point is to find a calculus book that proves this statement and have it open for this next part. I’m going to outline a series of thought-steps to consider as you read a proof.

### (1) Write down definitions.

Write down the definition of differentiability, and that of continuity. Study both. Play with examples of differentiable and continuous functions. See if all the differentiable functions you play with are also continuous. Try to play with some weird ones.

Examples: $f(x) = x^{2}$. Definitely differentiable. Definitely continuous. (I do suggest actually using the definitions here to really show that $f$ fits these.)

$f(x) = x+5$. Fits nicely. Try some nonpolynomials now.

$f(x) = \sin(x)$. Still good.

$f(x) = e^{x}$. Really nice. Love differentiating that guy.

As you’re playing with these, try to move between formally showing these functions fit the definitions and developing a visual picture of what it means for these functions to fit the definitions. (My high school calculus teacher, Andy Kohler, gave a nice intuitive picture of continuity—you shouldn’t have to pick up your pen to draw the function.)

This step helps you visualize your starting point and your destination. Every single time I develop a theorem, this is the process I go through. This isn’t always short either. I’ve spent weeks on this part—exploring connections between my start and my target to develop an intuition. Often, this leads me to a way to prove the desired theorem. Occasionally I manage to create an example that renders my statement untrue. This isn’t a trivial or unimportant part, so I implore you to temper any building frustration with the speed of this bit. However, since here we’re focused on reading proofs rather than writing them, we’ll likely assume we’re working with a true statement.

### (2) Now take a look at the proof.

A good proof should take you gently by the hand and guide you through the author’s reasoning in nice, comfortable steps of logic. The author shouldn’t require you to fill in gaps or make huge leaps, and especially never require you to just take something on faith.

The proof contains all the pieces we need for understanding. We just need to know how to read it. At some point in the proof, every single part of the IF should be invoked as a justification for a logical step. Find these parts.

“Because $f$ is differentiable, [STATEMENT]”

Study this part of the proof. Flag it. Do this for each time an IF condition is invoked. Make sure you can use the definition or condition invoked to make the leap the author does. Here I implore you to not just “pass over”. Really take the time to convince yourself this is true. Go back to the definition. Return to the proof. Again, this study isn’t necessarily quick.1

Doing this for each line in the proof will help you see what’s going on between IF and THEN.

### (3) Break things.

The last piece of advice I’ll give is one that was drilled into me by my graduate analysis professor at UTA, Dr. Barbara Shipman. Breaking things (or trying to) in mathematics is the best way to really cement your understanding.

Go back to all those points you flagged in the proof where the IF conditions were invoked. Now imagine you don’t have that condition anymore. The proof should fail in some way. Either you end up with a false conclusion (“if I don’t have X anymore, then I definitely cannot have Y”), or you just end up stuck (“if I don’t have X anymore, I can’t say anything about Y”.) This failure point helps you understand why that condition was necessary in the IF. Doing this for each condition in the proof helps you see the interplay among all the conditions, and showing you how and why all those parts were needed to get you to your conclusion.

None of this is trivial. Please don’t take the informal descriptions of this process to mean that you can breeze through and develop this skill as quickly as you memorized derivatives of functions. This is a skill that is developed over years. The reason I recommend using material you already know to study proofs is that it removes the challenge of learning new material while studying the “why”. There’s a reason mathematicians take calculus classes before real analysis courses (which spend time deeply developing and proving many things from calculus.) You’re familiar with how the subject works and that it works, then you can understand why it works.

This happens in engineering too. We flew a plane before we understood airflow deeply, but due to our understanding of airflow, we were able to advance flight technology far beyond what I imagine early aviators and inventors even thought possible. There’s a beautiful feedback loop between mathematics, formal proofs, and engineering. Mathematical proofs aren’t just the dry formalisms we use and throw away—they’re the keys to understanding why things work the way they do. Developing the skill to read and understand these arguments is not a waste of time for an engineer; it will help propel engineering forward.