A real-valued function 
defined on a convex subset 
is said to be quasi-concave
if for all real 
,
the set 
is convex. This is equivalent to saying that 
is quasi-concave if and only if its negative 
is quasi-convex.
Quasi-Concave Function
See also
Convex, Convex Function, Pseudoconcave Function, Pseudoconvex Function, Quasi-Convex FunctionThis entry contributed by Christopher Stover
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References
Borwein, J. and Lewis, A. Convex Analysis and Nonlinear Optimization: Theory and Examples. New York: Springer Science+Business Media, 2006.Cite this as:
Stover, Christopher. "Quasi-Concave Function." From MathWorld--A Wolfram Web Resource, created by Eric W. Weisstein. https://mathworld.wolfram.com/Quasi-ConcaveFunction.html