(264)

Equation (264) has a nontrivial solution ${C}_{1}\ne 0$ only if

$\mathrm{cos}\gamma l=0$ (265)

(265)

Substitution of Eq. (187) $z=\gamma l$ into Eq. (265) yields the eigenvalue equation

$\mathrm{cos}z=0$ (266)

(266)

which has the roots

$z=\frac{\pi}{2},3\frac{\pi}{2},5\frac{\pi}{2},\dots \mathrm{or}{z}_{j}=(2j-1)\frac{\pi}{2},j=1,2,3,\dots $ (267)

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