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\begin{equation*}\begin{array}{lcl}
P_{0} &=& \cos ^{2}\left( \frac{\phi }{2}\right) =\frac{1}{2}\left( 1+\cos\phi \right) \\
P_{1} &=& \sin ^{2}\left( \frac{\phi }{2}\right) =\frac{1}{2}\left( 1-\cos\phi \right)
\end{array}\end{equation*}
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