There are two mathematical constants called Feigenbaum constants. 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. 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, not just to the logistic map.

Both numbers are believed to be transcendental although they haven't been proven to be so.