pathological functions (‘disease’ function)

In mathematics, a pathological phenomenon is one whose properties are considered atypically bad or counterintuitive; the opposite is well-behaved. Often, when the usefulness of a theorem is challenged by counterexamples, defenders of the theorem argue that the exceptions are pathological.

A classic example is the Weierstrass function, which is continuous everywhere but differentiable nowhere.

Logic sometimes makes monsters. In former times when one invented a new function it was for a practical purpose; today one invents them purposely to show up defects in the reasoning of our fathers and one will deduce from them only that. If logic were the sole guide of the teacher, it would be necessary to begin with the most general functions, that is to say with the most bizarre. It is the beginner that would have to be set grappling with this teratologic museum. – Henri PoincarĂ©, 1899

In mathematics, counterexamples are often used to prove the boundaries of possible theorems. By using counterexamples to show that certain conjectures are false, mathematical researchers avoid going down blind alleys and learn how to modify conjectures to produce provable theorems.


Do not be deceived by the fancy methods of how mathematician prove their claims, for in most cases, mathematicians create theorems in such a way so that the theorems are provable. The intrinsic essence of math lies not in the proofs but in the process, intuition, motivation and inspiration of how such theorems are created.

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