For example, the critical points of the unit circle of equation "x"2 + "y"2 - 1 = 0 are (0, 1) and (0, -1) for the projection parallel to the "x"-axis, and (1, 0) and (-1, 0) for the direction parallel to the "y"-axis. If one considers the upper half circle as the graph of the function formula_3, then "x" = 0 is a critical point with critical value 1 due to the derivative being equal to 0, and x=-1 and x=1 are critical points with critical value 0 due to the derivative being undefined.