# A dirty little ditty on Finite Automata

This post builds on the previous post about Formal Languages.

### Some Formal Definitions

#### A Deterministic Finite Automata (DFA) is

• A set $\mathcal{Q}$ called “states”
• A set $\Sigma$ called “symbols” or “alphabet”
• A function $\delta_F:\mathcal{Q}\times\Sigma \to \mathcal{Q}$
• A designated state $q_0\in\mathcal{Q}$ called the start point
• A subset $F\subseteq\mathcal{Q}$ called the “accepting states”

The DFA is then often referred to as the ordered quintuple $A=(\mathcal{Q},\Sigma,\delta_F,q_0,F)$.

#### Defining how strings act on DFAs.

Given a DFA, $A=(\mathcal{Q}, \Sigma, \delta, q_0, F)$, a state $q_i\in\mathcal{Q}$, and a string $w\in\Sigma^*$, we can define $\delta(q_i,w)$ like so:

• If $w$ only has one symbol, we can consider $w$ to be the symbol and define $\delta(q_i,w)$ to be the same as if we considered $w$ as the symbol
• If $w=xv$, where $x\in\Sigma$ and $v\in\Sigma^*$, then $\delta(q_i, w)=\delta(\delta(q_i,x),v)$

And in this way, we have defined how DFAs can interpret strings of symbols rather than just single symbols.

#### The language of a DFA

Given a DFA, $A=(\mathcal{Q}, \Sigma, \delta, q_0, F)$, we can define “the language of $A$”, denoted $L(A)$, as ${w\in\Sigma^*|\delta(q_0,w)\in F}$.

### Some Examples, Maybe?

#### Example 1:

Let’s construct a DFA that accepts only strings beginning with a 1 that, when interpreted as binary numbers, are multiples of 5. So some examples of strings that would be in $L(A)$ are 101, 1010, 1111

### Some More Formal Definitions

#### A Nondeterministic Finite Automata (NFA) is

• A set $\mathcal{Q}$ called “states”
• A set $\Sigma$ called “symbols”
• A function $\delta_N:\mathcal{Q}\times\Sigma \to \mathcal{P}\left(\mathcal{Q}\right)$
• A designated state $q_0\in\mathcal{Q}$ called the start point
• A subset $F\subseteq\mathcal{Q}$ called the “accepting states”

The NFA is then often referred to as the ordered quintuple $A=(\mathcal{Q},\Sigma,\delta_N,q_0,F)$.

#### Defining how strings act on NFAs.

Given an NFA, $N=(\mathcal{Q}, \Sigma, \delta, q_0, F)$, a collection of states $\mathcal{S}\subseteq\mathcal{Q}$, and a string $w\in\Sigma^*$, we can define $\delta(\mathcal{S},w)$ like so:

• If $w$ only has one symbol, then we can consider $w$ to be the symbol and define $\delta(\mathcal{S},w):=\bigcup\limits_{s\in\mathcal{S}}\delta(s,w)$
• If $w=xv$, where $x\in\Sigma$ and $v\in\Sigma^*$, then $\delta(\mathcal{S}, w)=\bigcup\limits_{s\in\mathcal{S}}\delta(\delta(s,x),v)$

And in this way, we have defined how NFAs can interpret strings of symbols rather than just single symbols.