Given some point x′ on a contour of the function f(x), define the ‘better set’ B′ as that set of points which have the property: f(x)  f(x′). Can you frame definitions of quasi-concavity and of strict...

Given some point x′ on a contour of the function f(x), define the ‘better set’ B′ as that set of points which have the property: f(x)  f(x′). Can you frame definitions of quasi-concavity and of strict quasi-concavity of the function in terms of a property of this set B′?