Subset of recursively enumerable language as explained at: difference between recursive language and recursively enumerable language.
Set of all decision problems solvable by a Turing machine, i.e. that decide if a string belongs to a recursive language.
Or in other words: there is no Turing machine that always halts for every input with the yes/no output.
Every undecidable problem must obviously have an infinite number of "possibilities of stuff you can try": if there is only a finite number, then you can brute-force it.
Lists of undecidable problems.
Coolest ones besides the obvious boring halting problem:
If there are infinitely many inputs, we can always construct a (potentially exponentially huge) Turing machine that hardcodes the outcome for every possible input, so the problem is never undecidable.
The problem is of course deciding and proving the outcome for each possible input, notably as it is possible that calculation for some of the inputs may be independent from ZFC.
One of the most simple to state undecidable problems.
The reason that it is undecidable is that you can repeat each matrix any number of times, so there isn't a finite number of possibilities to check.
A:
- decidable problem is to a decision problem
- like a computable problem is to a function problem
The prototypical example is the Busy beaver function, which is the easiest example to reach from the halting problem.
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A recursive language (also known as a decidable language) is a type of formal language in the field of computer science and computational theory. Specifically, a recursive language is a set of strings over a given alphabet for which there exists a Turing machine that will accept every string in the language and will reject (or halt) every string that is not in the language.