Leading nonlinear eigenvalue shift in a Sturm-Liouville problem (source code)

= Leading nonlinear eigenvalue shift in a Sturm-Liouville problem

For $L(y;\lambda)=y^{m+1}$ near a normalized simple null mode $y_0$, put $y=\varepsilon y_0+O(\varepsilon^2)$ and $\lambda-\lambda_0=\varepsilon^m\mu$. Orthogonality to $y_0$ yields
$$
\mu=\int y_0^{m+2}\,dx+O(\varepsilon).
$$