= Bounded Green kernel for a radial Euler operator
{title2=$G(x,s)=-\min(x,s)/(3\max(x,s)^2)$}
For $L y=(x^2y')'-2y$ on $x>0$, choose solutions bounded at zero and infinity. The homogeneous <Cauchy-Euler equation> has basis $x,x^{-2}$. Joining the left-regular and right-regular solutions continuously and imposing $s^2[G_x]_{s^-}^{s^+}=1$ gives the displayed <Green's function> with ordinary integration measure $ds$. For bounded forcing,
$$
y(x)=-\frac1{3x^2}\int_0^x sf(s)ds-\frac x3\int_x^\infty\frac{f(s)}{s^2}ds.
$$
The integrals converge and give the unique bounded solution: no nonzero homogeneous combination is bounded at both ends. If $0\leq f\leq M$, negativity of the kernel gives $-M/2\leq y\leq0$. Differentiation cancels the two moving-endpoint terms, leaving
$$
y'(x)=\frac2{3x^3}\int_0^x sf(s)ds-\frac13\int_x^\infty\frac{f(s)}{s^2}ds,
$$
which implies $|y'(x)|\leq M/(3x)$. For constant forcing $f=M$, the bounded solution is $-M/2$; the endpoint conditions are boundedness, rather than zero endpoint values.
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