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Concavity
In mathematical analysis, concavity is a property of certain geometric figures, and in calculus, a property of certain graphs of functions.
Concave functions
In calculus, a differentiable function f is convex on an interval if its derivative function f ′ is increasing on that interval: a convex function has an increasing slope. Similarly, a differentiable function f is concave on an interval if its derivative function f ′ is decreasing on that interval: a concave function has a decreasing slope.
A function that is convex is often synonymously called concave upwards, and a function that is concave is often synonymously called concave downward.
For a twice-differentiable function f, if the second derivative, f ''(x), is positive (or, if the acceleration is positive), then the graph is convex (or concave upward); if the second derivative is negative, then the graph is concave (or concave downward). Points where concavity changes are inflection points.
If a convex (i.e., concave upward) function has a "bottom", any point at the bottom is a minimal extremum. If a concave (i.e., concave downward) function has an "apex", any point at the apex is a maximal extremum.
Contrary to the impression one may get from a calculus course, differentiability is not essential to these concepts; see convex.
In mathematics, a function f(x) is said to be concave on an interval [a, b] if, for all x,y in [a, b],
![\forall t\in[0,1],\ \ f(tx + (1-t)y) \geq tf(x) + (1-t)f(y).](a/../upload/math/df3d08823cf5978a51a65efc66991ca2.png)
Additionally, f(x) is strictly concave if
![\forall t\in[0,1],\ \ f(tx + (1-t)y) > tf(x) + (1-t)f(y).](a/../upload/math/a0ac3e47b0625fe09f2021e4ed8a0938.png)
A continuous function on C is concave if and only if
for any x and y in C.
Equivalently, f(x) is concave on [a, b] iff the function −f(x) is convex on every subinterval of [a, b].
If f(x) is twice-differentiable, then f(x) is concave iff f ′′(x) is non-positive. If its second derivative is negative then it is strictly concave, but the opposite is not true, as shown by f(x) = -x4.
A function is called quasiconcave iff there is an x0 such that for all x < x0,f(x) is non-decreasing while for all x > x0 it is non-increasing. x0 can also be 
, making the function non-decreasing (non-increasing) for all x. The opposite of quasiconcave is quasiconvex.
Concave polygons
In a concave polygon, some interior angle will be greater than 180°. The extension at that vertex of the line segment making up a side will pass through the interior of the polygon.
A concave polygon is often called re-entrant polygon (but in some cases the latter term has a different meaning).
See also
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