Formal Description
Consider an open subset U of the complex plane C. Let a be an element of U, and f : U \ {a} → C a meromorphic function. The point a is called an essential singularity of the function f if the singularity is neither a pole nor a removable singularity.
For example, the function f(z) = e1/z has an essential singularity at z = 0.
Read more about this topic: Essential Singularity
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