# Myhill-Nerode Theorem in Practice

When it comes to practical side of computer science, we often work with regular and context-free languages. Regular languages are most common for expressing syntax through the widely used regular expressions. Somewhat stronger context-free grammars dominate in the field of compilers and various language processors.

As we know from the Chomsky hierarchy of formal languages, the class of regular languages is a subset (or a sort-of a special case) of context-free language class. This means that every regular language can be described not only by finite automatons, regular expressions or regular grammars but also by context-free grammars, pushdown automatons and also (for the hard-core folks) a turing machine.

In theory this is fine, but it’s not very good for practice. I mean, if you were developing some app that requires syntax checking, would you rather develop some sort of lineary bounded automaton or use a library for regular expressions?

Sometimes we want to prove, that we don’t need to do magic to check the correct syntax of an email address. There are some ways of doing so. For example pumping theorem, which is fairly straight forward. But pumping lemma can be only used to prove, that a language is not regular. The lemma doesn’t provide sufficient condition for a language to be regular. That’s where the folks Myhill and Nerode come in!

The Myhill-Nerode theorem on the contrary provides necessary and sufficient condition for the language to be regular! Yay! But as you can imagine it’s not that simple :-P.

## Formal definition

Definition 1. Let Σ be an alphabet and ~ a equivalence relation on Σ*. Then relation ~ is right invariant if for all u, v, w ∈ Σ*:

• u ~ v <=> uw ~ vw

Definition 2. Let L be a language (not neccessarily regular) over Σ. We define relation ~L called prefix equivalence on Σ* as follows:

• u ~L v <=> ∀w ∈ Σ*: uw ∈ L <=> vw ∈ L

Theorem 1. (Myhill-Nerode) Let L be a language over Σ. Then these three statements are equivalent:

1. L is accepted by some deterministic finite automaton.
2. L is the union of some of the equivalence classes of a right invariant equivalence relation of finite index.
3. Relation ~L is of finite index.

## Informal description

The heart of this theorem is the fact that finite automaton has only a finite number of states. With this, any language that can be described by a finite automaton must consist of only finite number of string patterns. This is expressed by the right invariant relation with finite index.

The prefix equivalence is a form of the right invariant equivalence, but stronger. It’s tied to the respective language and says that if some strings are ~L-equivalent they both are included or excluded from the language after we extend them. The Myhill-Nerode theorem says, that a regular language always has a finite number of equivalence classes, i.e. there is only a finite number of word patterns that can be repeated through the string.

Example proof

Now a little example of how to show, that a language is not regular by using this theorem. Let’s take for instance the most classic context-free language of all times L = { anbn | n >= 0 }.

We’ll be interested in the third part of Myhill-Nerode theorem which states, that relation ~L is of finite index. We need to find a way of showing that it’s not and therefore the language is not regular (since Myhill-Nerode theorem is an equivalence).

In this case, the most elegant way is proof by contradiction. We will show, that L = { anbn | n >= 0 } is not reglar.

Proof 1. Suppose that L is a regular language. Let $i, j \in N$ be two natural numbers so $i \neq j$. Then consider words $u, v, w \in \Sigma^*$ and a sequence of words over $\Sigma^*$ a1, a2, …, ai.

Now let’s assign some values to strings u, v and w:

$u = a^i, v=a^j, c=b^i$.

According to the definition of prefix equivalence (definition 2),

$a^ib^i \in L \Leftrightarrow a^jb^i \in L$

we can see, that none of the words ai from the sequence are ~L-equivalent. The sequence is infinite, so the index of the relation will be infinite. But that’s a contradiction with the Myhill-Nerode theorem. Therefore, the language is not regular.

# Basic Computer Science

There is a couple of very basic definitions and axioms in computer science. I consider them to be very important, because everything that comes later is based on them. And if you don’t fully comprehend the basic stuff, it will be very hard to understand anything further. That’s why I decided to write a whole post on these trivial definitions.

## Alphabet

Yeah, I’m not kidding. There’s a formal definition for alphabet. And what’s worse: it actually makes sense. I just wonder what would a kinder-gardener say on this :-D. Anyway, here it goes:

Definition 1.1. An alphabet is a finite non-empty set of symbols.

Alphabets are usually denoted by Greek big sigma Σ. A set Σ can be referred to as alphabet when it’s not empty, but also not infinitely big.

• Σ = {a, b, c} is an alphabet
• Σ = {0, 1} is an alphabet
• Σ = {} is NOT an alphabet
• Σ = all integers; is NOT an alphabet

Content of an alphabet are symbols. A symbol is some indivisible or atomic unit that can have some meaning (not necessarily). For example, if Σ = {righteous, dude} were alphabet, righteous and dude would be considered to be atomic indivisible elements (even though they contain multiple characters). Some folks also choose to omit the finite word from the definition. But that’s for some very advanced stuff and I’m guessing that finite alphabets will be more than sufficient for us.

## String

Another fundamental thing is a string. String or a word is a sequence of some symbols (the order is important). Strings usually contain symbols from only one alphabet.  In that case we say string $x$ over $\Sigma$ is a finite sequence of symbols from $\Sigma$. The formal definition is this:

Definition 1.2. Let $\Sigma$ be an alphabet.

1. empty string $\epsilon$ is string over $\Sigma$
2. if $x$ is a string over $\Sigma$ and $a \in \Sigma$ is a symbol from $\Sigma$, then $xa$ is a string over $\Sigma$
3. y is string over $\Sigma$ only when we can derive it by applying rules 1 and 2.

There are some more basic definitions like string length, concatenation, reversation that are useful and important, but they might be a little intimidating at first, so let’s skip ahead to languages.

## Language

What is a formal language? We need to define $\Sigma^*$ first. $\Sigma^*$ is a set of all existing strings over alphabet $\Sigma$. For example, let $\Sigma = \{0, 1\}$, then $\Sigma^* = \{\epsilon\} \cup \{$ all positive binary numbers $\}$.

Then a formal language L over alphabet $\Sigma$ is a subset of $\Sigma^*$. Formally said, $L \subseteq \Sigma^*$. Basically, language is a set of strings. In context of programming languages, a language would be a set of all possible programs in that programming language. You see, that in most practical cases the set will be infinite (as it is in the programming language case). So, it’s not very practical to describe languages by enumerating all possible sentences of the language. One way of describing a language is with things called formal grammars. But we’ll get on to that later.

# Introduction to Computer Science

In the upcoming semester I’ll be taking class called Theoretical Computer Science. Which is said to be the hardest thing you can attend here at BUT. Only half the people pass the bar every year. It’s brutal. And since I’d really like to be a part of the lucky half, I thought I could dig into the theory a little earlier and see how bad it is.

So Computer Science, right? What the hell is all that about? Theoretical computer science or theoretical information technology (as referred by some people) is a formal foundation for the things we like to call computers. What computers do? They essentially compute stuff. Current computer hardware is built and programmed to work with numbers. Who cares about a bunch of numbers? But the numbers represent some sort of information. In broader terms, computers work with information. A computer takes some information and transforms it into another information. What Wikipedia says:

Computer science deals with the theoretical foundations of information, computation, and with practical techniques for their implementation and application. Computer scientists invent algorithmic processes that create, describe, and transform information and formulate suitable abstractions to model complex systems.

Right, so we have information. How do people work with these anyway? People share their thoughts though languages. We talk and sometimes listen, we also read and sometimes write. Other people like to draw, play charades or sing. In all of these cases the information is encoded into some sort of language that others can understand. Sure, we’re people, it comes naturally to us, but what about the machines? A coffeemaker will most certainly not learn to talk to you or even understand your needs. Machines are dumb. That’s where we (the nerdy guys from engineering department) come in with formal languages — a substantial part of computer science. By formalizing common languages we make the machines understand our instructions.

Ever heard of a programming language? Programming language is basically a sequence of instructions we use when we tell the computers what to do. It’s a language that we use when we talk to the computers. It sounds a little weird, but that’s it. Computer science sets up some ground rules so the computers can algorithmically analyze the instructions and process them. It explores possibilities of computers — what can be processed by a computer? Is there anything that computers can’t solve, why? Bunch of interesting stuff.

Interesting, but sometimes very hard to understand. I wanted to put some basics here too, but I don’t want to scare you off too early. I got stuck for a while with the very basics at first (I figured it out eventually). The math will come in a stand-alone post shortly after. Stay tuned ;-).