There are two mathematical constants called Feigenbaum constants, named after mathematician Mitchell Feigenbaum. Both express ratios in a bifurcation diagram.
is the ratio between successive bifurcation intervals, or between the diameters of successive circles on the axis of the Mandelbrot set. Feigenbaum originally related this number to the period-doubling bifurcations in the logistic map, but also showed it to hold for all one-dimensional maps displaying a single hump. As a consequence of this generality, every chaotic system that corresponds to this description will bifurcate at the same rate. Feigenbaum's constant can be used to predict when chaos will arise in such systems before it ever occurs.
The second Feigenbaum constant,
,
is the ratio between the width of a tine and the width of one of its two subtines (except the tine closest to the fold).
These numbers apply to a large class of dynamical systems. Both numbers are believed to be transcendental although have not been proven to be so.