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The Feigenbaum period-doubling map is the universal, even, quadratic-critical fixed map associated with repeated period-doubling bifurcations. One normalization isComposing twice and rescaling therefore reproduces the same graph. This self-similarity is the mechanism behind a common spatial scaling in otherwise different one-dimensional maps with a quadratic maximum.^[ref]

The quadratic maximum means $g^{\prime }(0)=0$ and $g^{\prime \prime }(0)\ne 0$. Twice applying the chain rule at zero givesThe term involving $g^{\prime }(0{)}^2$ vanishes, and cancellation is legitimate because the maximum is nondegenerate. Thus the endpoint derivative is determined by the spatial scaling.

Writing $\gamma =-\beta >0$ makes the successive central intervals $[-{\gamma }^n,{\gamma }^n]$ shrink by the factor $\gamma$. Their iterates generate the Feigenbaum attractor; nonuniform interval lengths are encoded by the Feigenbaum geometric potential. The constant $\beta$ here describes spatial scaling, rather than the spacing of successive bifurcation parameter values.

Mitchell Feigenbaum discovered quantitative universality in period-doubling sequences of nonlinear maps. For a class of smooth maps with a quadratic maximum, successive parameter intervals shrink with a limiting ratio near $4.669$, one of the Feigenbaum constants.^[ref]

The striking result is independence from many details of the map, within the stated universality class. The Feigenbaum renormalization fixed point expresses that repeated rescaling approaches one common structure. A different shape of the critical maximum can belong to a different class, so the numerical constant is not universal for every nonlinear system.