# Theory of Computation: Question Set – 24

#### Can a Turing machine recognize an uncountable language?

No, a Turing machine can only recognize countable languages, which are languages whose strings can be enumerated by a Turing machine. Uncountable languages, such as the set of all real numbers, cannot be recognized by a Turing machine.

#### What is the difference between a decider and a recognizer in Turing machine theory?

A decider is a Turing machine that halts and accepts or rejects its input on all possible inputs, while a recognizer is a Turing machine that halts and accepts its input on all strings in the language it recognizes, but may loop indefinitely or reject other strings.

#### How can you simulate a multi-tape Turing machine on a single-tape Turing machine?

A multi-tape Turing machine can be simulated on a single-tape Turing machine by encoding the contents of all tapes onto the single tape in a way that preserves their relative positions. This can be done using a special symbol to separate the different tapes, and by using other special symbols to represent the head positions and states of the original machine.

#### Can a Turing machine recognize a non-context-free language?

Yes, a Turing machine can recognize non-context-free languages by using its unbounded memory to keep track of the context of the input string. However, the recognition process may require an infinite number of steps, and the machine may not always halt on certain inputs.

#### What is the Church-Turing thesis?

The Church-Turing thesis is the informal hypothesis that any function that is effectively computable can be computed by a Turing machine, or equivalently, by any other model of computation that is as powerful as a Turing machine. The thesis is widely accepted as true, although it is difficult to prove formally, and it is subject to some interpretations and limitations.

#### What is the difference between a Turing machine and a finite automaton?

A Turing machine is a more powerful model of computation than a finite automaton. While a finite automaton can only recognize regular languages, a Turing machine can recognize context-free and recursively enumerable languages. The main difference between the two models is that a Turing machine has an unbounded tape, which allows it to store an infinite amount of information.

#### Can a Turing machine simulate a computer?

Yes, a Turing machine can simulate a computer by reading in the binary representation of the program and the input, and performing the computations specified by the program. In practice, this may not be practical or efficient, but it demonstrates the theoretical power of Turing machines.

#### Can a Turing machine recognize an undecidable language?

No, a Turing machine cannot recognize an undecidable language, which is a language for which there is no algorithm that can determine whether a given string is in the language or not. Turing machines can only recognize recursively enumerable languages, which are languages for which there is an algorithm that can list all the strings in the language.