= Symmetric Robin strip transform
{title2=$D(k)=(k-\gamma)-(k+\gamma)e^{k\ell}$}
For a decaying harmonic function on a semi-infinite strip with equal outward <Robin boundary conditions> and reflection-symmetric end data, the top and bottom value traces agree. The <global relation> and its reflected spectral companion express their transforms through $D(k)$. With $H(k)=\tfrac12\int_0^\ell e^{ky}g(y)dy$, the relation is $\psi(-ik)+\psi(ik)=2H(k)/D(k)$. Real zeros of $D$ require compatible data; unrestricted negative Robin parameters do not guarantee a decaying solution.
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