How to calculate the "Bounds" of the Mandelbrot set?

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I would like to know, how to calculate the "Bound numbers" of the Mandelbrot set. So I mean the last number c that can be substitutet in $ z, z^{2} + c, \left( z^{2} + c \right)^{2} + c, \left( \left( z^{2} + c \right)^{2} + c \right)^{2} + c, \dots $ before the series diverges. How to calculate that?