Bifurcation Velocity (first Feigenbaum constant) :
The "bifurcation velocity" (d)
governs the geometric onset of chaos via period-doubling
in iterative sequences (with respect to some parameter
which is used linearly in each iteration, to damp a given function
having a quadratic maximum).
This universal constant was unearthed in October 1975 by
J. Feigenbaum (b.1944).
Reduction Parameter (second Feigenbaum constant) :
The related "reduction parameter" (a) is the
second Feigenbaum constant...
Feigenbaum Constant :
(Eric W. Weisstein)
Mathsoft (Steve Finch)
Why 4.669 is famous (18:54)
by Ben Sparks (Numberphile, 2017-01-16).