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# Long description

The figure consists of a perspective drawing of a three-dimensional set of Cartesian axes with a smooth surface sketched on it. The complex plane is drawn as horizontal, with axes labelled x and y. The x-axis points out of the page and to the left and the y-axis points out of the page and to the right from the origin. The vertical axis is labelled s. A smooth surface shaded darker on top and lighter underneath starts like a sheet with the top edge attached along the y-axis. At this point it is flat. As the x-coordinate increases, the surface rises up in a saddle shape above the x-axis, and dips down on either side. The vertical cross-section of the surface parallel to the s-y plane is shaped like an inverted parabola, symmetrical about a vertical line through the x-axis. On the surface a point is marked with a solid dot and labelled with its coordinates: capital P equals, open bracket, 2 comma 1 comma 2, close bracket.