= Dirichlet Green function inside a hemisphere
{c}
{title2=$G_{\rm hemi}(x,y)=G_D(x,y)-G_D(x,\overline y)$}
For the upper half of a three-dimensional ball, let $\overline y$ be the reflection of an interior source $y$ in the equatorial plane. The <Dirichlet Green function inside a sphere>, evaluated at the source and its reflection and then subtracted, vanishes on both the spherical cap and flat equatorial disc. Reflection makes the source distances and image distances agree on the plane. Only the original source is in the upper half-ball; the reflected source is in the lower half-ball, and both inversion images are outside the ball. Thus the difference has precisely the required <Dirac delta distribution> and <Dirichlet boundary conditions>.
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