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# Long description

This figure consists of a perspective drawing of a three-dimensional set of Cartesian axes. The complex plane is drawn as horizontal, with axes labelled x and y. On the page, the x-axis slopes up and to the right in the positive direction, while the y-axis slopes up and to the left. The third axis is drawn as vertical on the page and is labelled s. It is used to represent the value of the real-valued function u, defined in the text, for each pair of values of x and y. The surface corresponding to the graph of the function u is shaded on the diagram. In the upper-left, lower-left and lower-right quadrants of the x-y plane, the function u takes the value zero, so the x y plane itself is shaded in these quadrants. In the upper-right quadrant, the value of the function at each point on the x-y plane is the minimum of the x and y coordinates of the point. The surface corresponding to the graph in the upper-right quadrant looks a little like a square pyramid, with the origin at one corner of its base, and one of its slant edges sloping up from the origin at an angle of 45 degrees, above the line y equals x on the x-y plane. To emphasise this shape, the slanting face of the pyramid that slopes up from the y-axis is shaded darker than the slanting face that slopes up from the x-axis.