Verify Eq. (4.18).
Substitute Eq. (4.2) into Eq. (4.1):
Multiply numerator and denominator by V and note that
k_{\infty} = \frac{V_{f}{\int_{0}^{\infty}}\nu \Sigma_{f}^{f}(E)\varphi(E)d E}{V_{f}{\int_{0}^{\infty}}\Sigma_{a}^{f}(E)\varphi(E)d E+V_{c}\int_0^{\infty}\Sigma_{a}^{c}(E)\varphi(E)d E+V_{s t}\int_0^{\infty}\Sigma_{\alpha}^{st}(E)\varphi(E)d E}