A function that has a natural boundary.
Lacunary Function
See also
Natural BoundaryThis entry contributed by Jonathan Deane
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References
Ash, R. B. Ch. 3 in Complex Variables. New York: Academic Press, 1971.Referenced on Wolfram|Alpha
Lacunary FunctionCite this as:
Deane, Jonathan. "Lacunary Function." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/LacunaryFunction.html