20.2 fit \(a\) from perturbative expansion

\[ (\Delta E_{2\phi_0} ) E_{\phi_0}/2= \frac{-2\pi a_0}{L^3} \left[ 1 +c_1 \frac{a_0}{L} + c_2 \frac{a_0^2}{L^2} \right] \] \(c_1=- 2.837297\) and \(c_2= 6.375183\)

\[\begin{gather} \chi^2/d.o.f.=0.368107 \\ P[0]=-0.549385\pm (0.0085) \\ \end{gather}\] {\[\begin{gather} C=\begin{pmatrix} 7.3e-05\\ \end{pmatrix} \end{gather}\]}