# Computability Theory - Decidability(Decidable Languages)

Computability Theory - Decidability - Decidable Languages

## Intro

We know Church-Turing thesis essentially says that the notion of algorithm are equivalent to Turing machine(TM). A TM can simulate a computer by simulating the basic operations like addition, multiplication, division, etc. A computer can also simulate a TM to keep track of tape contents by using programming languages like Python.

**If a TM cannot solve a problem, that means a computer can also not solve that problem**. When we say "cannot

__a problem" in previous sentence. The words__

**solve**__means there is no__

**solve****decider**for a given Language $L$.

Here, nodeciderdoesn't means norecognizer.

Can Turing machine answer questions about what other machines(DFAs, for example) can do/be solved?

- We can build up to Turing machines which allows us to show that a problem can be solved.

That's what what this article will show next.

## Decidable Languages

### Representation / Encoding

A string representation/encoding of machine $M$ can be written as $\langle M \rangle$.

### Acceptance problem for DFA

The ** acceptance problem** for DFAs of testing whether a particular DFA accepts a given string can be expressed as a language, $A_{DFA}$.

$$ A_{DFA} = \{ \langle B,w \rangle \mid B \text{ is a DFA that accepts input string } w\} $$

- $A$ states for an
.*acceptance problem* - $\langle B,w \rangle$ means whether some string encoding of a DFA $B$ accepts an input $w$.

Overall, it means the problem of **testing whether a DFA $B$ accepts an input $w$** is the same as the problem of **testing whether $\langle B,w \rangle \in A_{DFA}$**.

__Theorem:__

$A_{DFA}$ is decidable. (In another words, a TM exists for $A_{DFA}$ that always halts)

__Proof:__

$M$ = "On input $\langle B,w \rangle$, where $B$ is a DFA and $w$ is a string:

- Simulate $B$ on input $w$.
- If the simulation($B$) ends in an accept state, $M$
*accepts*. If it ends in a non-accept state, $M$*reject*."

Note: $M$ is a Turing machine, and $M$ is adeciderfor $A_{DFA}$. It's important to understand only a language can bedecidable, a machine is adecider.

### Acceptance problem for NFA

$$ A_{NFA} = \{ \langle B,w \rangle \mid B \text{ is a NFA that accepts input string } w\} $$

__Theorem:__

$A_{NFA}$ is decidable.

__Proof:__

$N$ = "On input $\langle B, w \rangle$, where $B$ is a NFA and $w$ is a string:

- Convert NFA $B$ to an equivalent DFA $C$.
- Run the Turing machine $M$ for $A_{DFA}$ on $\langle C, w \rangle$
- If $M$ accepts,
*accept*; otherwise,*reject*." (Output whatever the Turing machine $M$ says)

Note 01: In step #2, it is important to know when we send something to machine $M$. Let's for example $\langle X, x \rangle$, $X$

has to be a DFA(NOT an NFA or something else). It is because we defined that in problem "Acceptance problem for DFA".Note 02: The reason why we need convert a DFA to a NFA is because a NFA may includes $\epsilon$ transition. So, it may ends up with a loop and run forever.

### Acceptance problem for REX

$$ A_{REX} = \{ \langle R,w \rangle \mid R \text{ is a regular expression that generates string } w\} $$

__Theorem:__

$A_{REX}$ is decidable.

__Proof:__

$P$ = "On input $\langle R, w \rangle$, where $R$ is a regular expression and $w$ is a string:

- Convert regular expression $R$ to an equivalent NFA $A$.
- Run TM $N$ on input $\langle A, w \rangle$
- If $N$ accepts,
*accept*; if $N$ rejects,*reject*."

### Emptiness problem for DFA

$$ E_{DFA} = \{ \langle A \rangle \mid A \text{ is a DFA and } L(A) = \empty\} $$

- $E$ states for an
.*emptiness problem*

Overall, it means **whether a DFA $A$ accepts nothing**(no accepting string in language $A$).

__Theorem:__

$E_{DFA}$ is decidable.

__Proof:__

$T$ = "On input $\langle A \rangle$, where $A$ is a DFA:

- Mark the start state of $A$.
Repeat until no new states get marked:

- Mark any state that has a transition coming into it from any state that is already marked.

- If no accept state is marked,
*accept*; otherwise,*reject*."

Note: A naive way to think about this problem is by looking at, if DFA $A$ has a final states. That is

NOT true. A DFA can have aunreachablefinal states, which has a final states but accepting nothing.A right way to think of the problem is that if there exists a set of transitions allow us to reach final state.

### Equality problem for DFA

$$ EQ_{DFA} = \{ \langle A,B \rangle \mid A \text{ and } B \text{ are DFAs and } L(A) = L(B)\} $$

__Theorem:__

$EQ_{DFA}$ is decidable.

__Proof:__

$$ L(C) = (L(A)\cap \overline{L(B)}) \cup (\overline{L(A)} \cap L(B)) $$

$F$ = "On input $\langle A,B \rangle$, where $A$ and $B$ are DFAs:

- Construct DFA $C$ as described.
- Run TM $T$ on input $\langle C \rangle$.
- If $T$ accepts,
*accept*. If $T$ rejects,*reject*."

It's important to understand the idea behind the proof algorithm:

$$ \langle A,B \rangle \in EQ_{DFA} \iff L(A) = L(B) \\ \iff L(A) \subseteq L(B) \text{ AND } L(B) \subseteq L(A) $$

We also know:

$$ L(A) \subseteq L(B) \iff L(A) \cap \overline{L(B)} = \empty \\ L(B) \subseteq L(A) \iff L(B) \cap \overline{L(A)} = \empty $$

That's how we got $L(C)$:

$$ L(C) = (L(A)\cap \overline{L(B)}) \cup (\overline{L(A)} \cap L(B)) $$

### Acceptance problem for CFG

$$ A_{CFG} = \{ \langle G,w \rangle \mid G \text{ is a CFG that accepts input string } w\} $$

__Theorem:__

$A_{CFG}$ is decidable.

This implies $A_{PDA}$ is also decidable, since we can convert a CFG into a PDA.

__Proof:__

$S$ = "On input $\langle G,w \rangle$, where $G$ is a CFG and $w$ is a string:

- Convert $G$ to an equivalent grammer $C$ in Chomsky normal form(CNF).
- If $w=\epsilon$, and $S \to \epsilon$ is a rule in $C$,
*accept*; If $S \to \epsilon$ is a not rule in $C$,*reject*. - let $n$ be the length of string $w$.
- Generate all possible derivations of $2n-1$.
- If any of the dervations result in $w$,
*accept*; otherwise,*reject*"

### Acceptance problem for CFL

$$ A_{CFL} = \{ \langle G,w \rangle \mid G \text{ is a CFL that accepts input string } w\} $$

__Theorem:__

$A_{CFL}$ is decidable.

__Proof:__

$M_G$ = "On input $w$:

- Run TM $S$ on input $ \langle G,w \rangle$.
- If this machine accepts,
*accept*; if it rejects,*reject*"

### Emptiness problem for CFG

$$ E_{CFG} = \{ \langle G \rangle \mid G \text{ is a CFG and } L(G) = \empty\} $$

__Theorem:__

$E_{CFG}$ is decidable.

__Proof:__

$R$ = "On input $\langle G \rangle$, where $G$ is a CFG:

- Mark all terminal symbols in $G$.
Repeat until no new variables get marked:

- Mark any variable $A$ where $G$ has a rule $A \to U_1 U_2 ... U_k$ and each symbol $U_1, ..., U_k$ has already marked.

- If the start variable is not marked,
*accept*; otherwise,*reject*."

### Chomsky Hierarchy

### Summary

TM | Languages / Problems |
---|---|

$M$ | $A_{DFA} = \{ \langle B,w \rangle \mid B \text{ is a DFA that accepts input string } w\}$ |

$N$ | $A_{NFA} = \{ \langle B,w \rangle \mid B \text{ is a NFA that accepts input string } w\}$ |

$P$ | $A_{REX} = \{ \langle R,w \rangle \mid R \text{ is a regular expression that generates string } w\}$ |

$T$ | $E_{DFA} = \{ \langle A \rangle \mid A \text{ is a DFA and } L(A) = \empty\}$ |

$O$ | $ALL_{DFA} = \{ \langle A \rangle \mid A \text{ is a DFA and } L(A) = \Sigma^{*}\}$ |

$F$ | $EQ_{DFA} = \{ \langle A,B \rangle \mid A \text{ and } B \text{ are DFAs and } L(A) = L(B)\}$ |

$S$ | $A_{CFG} = \{ \langle G,w \rangle \mid G \text{ is a CFG that accepts input string } w\}$ |

$M_G$ | $A_{CFL} = \{ \langle G,w \rangle \mid G \text{ is a CFL that accepts input string } w\}$ |

$R$ | $E_{CFG} = \{ \langle G \rangle \mid G \text{ is a CFG and } L(G) = \empty\}$ |