Feigenbaum constants

For a generic quadratic period-doubling cascade, the Feigenbaum constants are the limiting parameter ratio $\delta$ and spatial ratio $\alpha$. With $s_n$ the successive superstable period-$2^n$ parameters and $d_n=f_{s_n}^{2^{n-1}}(0)$ the signed central-orbit spacing,The positive convention gives $\delta \simeq 4.669201609$ and $\alpha \simeq 2.502907875$. The orientation-reversing spatial scale at the Feigenbaum renormalization fixed point is $a=-1/\alpha$.

// Target: dynamical-systems.bigb