11.4 3 parameters fit

$$\frac{k}{m} \cot{ \delta}=\frac{1}{am}+\frac{r_0 m }{2}\frac{k^2}{m^2}+ \frac{P_2 r_0^3 k^4}{m}$$

$P[0]=am$ , $P[1]=r_0m$ , $P[2]=P_2$

$$\begin{gather} \chi^2/d.o.f.=5.58424 \\ P[0]=-0.120224\pm (0.0028) \\ P[1]=-17.0848\pm (0.64) \\ P[2]=-0.000368459\pm (3.7e-06) \\ \end{gather}$$ { $$\begin{gather} C=\begin{pmatrix} 8.03e-06& -0.282& -0.283\\ -0.282& 0.413& 0.743\\ -0.283& 0.743& 1.35e-11\\ \end{pmatrix} \end{gather}$$ }