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In page Mandelbrot set:

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For an integer d, these sets are connectedness loci for the Julia sets built from the same formula. The full cubic connectedness locus has also been studied; here one considers the two-parameter recursion z z 3 + 3 k z + c {\displaystyle z\mapsto z^{3}+3kz+c} , whose two critical points are the complex square roots of the parameter k. A parameter is in the cubic connectedness locus if both critical points are stable.[1] For general families of holomorphic functions, the boundary of the Mandelbrot set generalizes to the bifurcation locus.[citation needed]