讲解一下这道题---**Extracted Content:** **Question:** 16. 如图, 已知点 M (a, b), b < 0 为抛物线 y=x²-2x-3 上的动点, 点 N 是以点 M 为圆心, 1 为半径的圆上的动点, 点 A (1, 0), 则线段 AN 的最小值为 \_\_\_\_\_\_\_\_ . **Mathematical Formula/Equation:** y = x² - 2x - 3 **Chart/Diagram Description:** Type: Coordinate plane with a parabola and a circle. Main Elements: * Coordinate Axes: X-axis and Y-axis labeled 'x' and 'y' respectively, intersecting at the origin O. * Parabola: A curve opening upwards, located below and crossing the x-axis. * Points: * O: Origin (0, 0). * A: Labeled on the positive x-axis, corresponding to the point (1, 0) as given in the text. * M: Labeled within the lower portion of the parabola's opening, indicated as the center of the circle. * N: Labeled on the circumference of the circle. * Circle: Centered at point M. * Labels and Annotations: O, A, M, N, x, y are labeled on the diagram. * Relative Position and Direction: Point A is on the x-axis to the right of O. The parabola has its vertex below the x-axis. Point M is shown in the region where both x > 0 and y < 0. The circle centered at M is depicted with point N on its edge.