Reflection-symmetric trials for a linearly weighted Rayleigh quotient ($\frac{{\int }_0^L(y^{\prime }{)}^{2}dx}{{\int }_0^{L}x{y}^{2}dx}=\frac{2}{L}\frac{{\int }_0^L(y^{\prime }{)}^{2}dx}{{\int }_0^L{y}^{2}dx}$)

For a nonzero trial function with $y(0)=y(L)=0$ and $y(L-x)=y(x)$, substituting $L-x$ for $x$ proves ${\int }_0^{L}x{y}^{2}dx=(L/2){\int }_0^L{y}^{2}dx$. This gives the displayed Rayleigh quotient. The Poincaré inequality on an interval bounds it below by $2{\pi }^2/L^3$, with equality for a nonzero multiple of $\sin (\pi x/L)$. Thus this sine is the best reflection-symmetric trial for $-y^{\prime \prime }=\lambda xy$, while the unrestricted lowest eigenfunction need not be reflection-symmetric. The polynomial $x(L-x)$ gives $20/L^3$, only a factor $10/{\pi }^2$ above the sine's value. Both are upper bounds on the unrestricted lowest eigenvalue by the Rayleigh-Ritz method.