A function is called strictly concave if. for any t in (0,1) and x ≠ y. For a function f: R → R, this definition merely states that for every z between x and y, the point (z, f (z)) on the graph of f is above the straight line joining the points (x, f (x)) and (y, f (y)). Any affine function is both (non-strictly) concave and convex. 2  The sine function is concave on the interval. 3  The function, where is the determinant of a nonnegative-definite matrix B, is concave. 4  Practical example: rays bending in computation of radiowave attenuation in the atmosphere.