21. The first two terms of the orbit of (0,0) under the Mandelbrot map with control parameters (a,b) are (0,0) and (a,b). Since the orbit of (0,0) is preperiodic, so is the orbit of (a,b). Consequently, the point (a,b) does not escape to infinity under the Mandelbrot map. That is, (a,b) belongs to the Julia set.
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