A singular point 
for which 
is not differentiable
for any integer 
.
Essential Singularity
See also
Picard's Great Theorem, Pole, Removable Singularity, Singularity, Weierstrass-Casorati TheoremExplore with Wolfram|Alpha
References
Knopp, K. "Essential and Non-Essential Singularities or Poles." §31 in Theory of Functions Parts I and II, Two Volumes Bound as One, Part I. New York: Dover, pp. 123-126, 1996.Krantz, S. G. "Removable Singularities, Poles, and Essential Singularities." §4.1.4 in Handbook of Complex Variables. Boston, MA: Birkhäuser, p. 42, 1999.Referenced on Wolfram|Alpha
Essential SingularityCite this as:
Weisstein, Eric W. "Essential Singularity." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/EssentialSingularity.html