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Many nonconstructive proofs assume the non-existence of the thing whose existence is required to be proven, and deduce a contradiction. The non-existence of the thing has therefore been shown to be logically impossible, and yet an actual example of the thing has not been found.
The term "existence proof" is often used as a synonym for "nonconstructive proof", although this is not strictly accurate, as both constructive and nonconstructive proofs can be used to prove existence. See existence theorem.
Some examples of nonconstructive proofs
An example is the following proof of the theorem "There exist irrational numbers a and b such that ab is rational."
- Recall that is irrational, and 2 is rational. Consider the number . Either it is rational or it is irrational.
- If it is rational, then the theorem is true, with a and b both being .
- If it is irrational, then the theorem is true, with a being and b being , since
A constructive proof of this theorem would leave us knowing values for a and b.
Since we don't know this (because we don't know whether q is irrational), this proof is nonconstructive. (The statement "Either q is rational or it is irrational", from the above proof, is an instance of the law of excluded middle, which is not valid within a constructive proof.) (Side note: As it happens, one can prove that q is irrational using the Gelfond-Schneider theorem, proving the above theorem in a different manner and giving an actual example; however, as this is not done in the above proof, the above proof remains nonconstructive)
Nearly every proof which invokes the axiom of choice is nonconstructive in nature because this axiom is fundamentally nonconstructive.
According to the philosophical viewpoint of constructivism, nonconstructive proofs constitute a different kind of proof from constructive proofs. Supporters of this view consider nonconstructive existence to be a weaker form of existence than its constructive counterpart. Some constructivists deny the validity of nonconstructive proof altogether.
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