= Side localization of modified Helmholtz plane waves
{title2=$e^{\eta(n\cdot r-1)+ip\,t\cdot r},\quad\eta^2-p^2=m^2$}
On a square, a normalized <modified Helmholtz adjoint plane wave> with positive normal growth toward one side has magnitude one there, magnitude $e^{-2\eta}$ on the opposite side, and an adjacent-side magnitude integral bounded by $1/\eta$. Thus large tangential spectral frequencies suppress remote-side normal-trace coefficients. Paired <sine collocation of square modified Helmholtz global relations> further cancels adjacent-side unknown traces exactly.
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