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Computing (FOLDOC) dictionary
computability theory
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mathematics The area of theoretical computer science
concerning what problems can be solved by any computer.
A function is computable if an algorithm can be implemented
which will give the correct output for any valid input.
Since computer programs are countable but real numbers are
not, it follows that there must exist real numbers that
cannot be calculated by any program. Unfortunately, by
definition, there isn't an easy way of describing any of them!
In fact, there are many tasks (not just calculating real
numbers) that computers cannot perform. The most well-known
is the halting problem, the busy beaver problem is less
famous but just as fascinating.