= Green kernel with a vanishing Wronskian at an interior singular point
{c}
{title2=$G(x,\xi)=u(\min(x,\xi))v(\max(x,\xi))/W(\xi)$}
The usual <Green function> formula for a monic second-order equation requires a nonzero <Wronskian> at the source. At an interior <singular point> it may fail to define an ordinary inverse for arbitrary forcing. For example, for $Ly=y''-(1+x)y'/x+y/x$ on $[-1,1]$, choose $u=1+x$ and $v=e^x-e(1+x)/2$. They satisfy the left and right <Dirichlet boundary conditions>, and $W=uv'-u'v=xe^x$. The displayed kernel is valid for $\xi\ne0$ and has unit jump in its first $x$ derivative. Its $1/\xi$ behaviour prevents unrestricted integration through zero. For smooth forcing $f(x)=xq(x)$, the product $G(x,\xi)f(\xi)$ has a removable singularity, and its integral gives the unique solution extending with two continuous derivatives through zero. That matching condition matters: merely requiring one continuous derivative allows additional piecewise homogeneous solutions. A smooth forcing with $f(0)\ne0$ cannot have a solution with two continuous derivatives there, as expansion of $xy''-(1+x)y'+y=xf$ shows.
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