15.2 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 $c_2= 6.375183$

$$\begin{gather} \chi^2/d.o.f.=2.58281 \\ P[0]=-0.576923\pm (0.0062) \\ \end{gather}$$ { $$\begin{gather} C=\begin{pmatrix} 3.82e-05\\ \end{pmatrix} \end{gather}$$ }