In the field of computational complexity theory, the concept of decidability plays a crucial role in understanding the limits and possibilities of solving computational problems. Decidability refers to the property of a problem being solvable by an algorithm, meaning that there exists a procedure that can determine the correct answer for any given instance of the problem. In other words, a problem is decidable if there exists a Turing machine or an equivalent computational model that can halt and provide the correct answer for every input.

To formally define decidability, we need to introduce the notion of a decision problem. A decision problem is a computational problem that has a yes-or-no answer, where the goal is to determine whether a given input satisfies a certain property or condition. For example, the problem of determining whether a given number is prime or composite is a decision problem.

Now, a problem is said to be decidable if there exists an algorithm, or a Turing machine, that can correctly decide the problem for all possible inputs. This means that the algorithm will always halt and provide the correct answer, either "yes" or "no", for any input instance of the problem. In other words, there is an algorithm that can solve the problem in a finite amount of time.

Decidability is a fundamental concept in computational complexity theory because it helps us understand the inherent limitations of computation. Not all problems can be solved by an algorithm, and decidability allows us to distinguish between problems that are solvable and those that are not. For example, the halting problem, which asks whether a given Turing machine halts on a given input, is undecidable. This means that there is no algorithm that can correctly decide the halting problem for all possible inputs.

On the other hand, many important problems in computer science and cybersecurity are decidable. For instance, the problem of determining whether a given graph is connected can be solved by a simple algorithm that performs a depth-first search or a breadth-first search. Similarly, the problem of determining whether a given string is a valid regular expression can be solved by constructing a deterministic finite automaton that recognizes the language defined by the regular expression.

In the context of computational complexity theory, a problem is said to be decidable if there exists an algorithm that can correctly determine the answer for all possible inputs. Decidability is a fundamental concept that helps us understand the limits and possibilities of computation, and it allows us to distinguish between problems that can be solved by an algorithm and those that cannot.

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