Is the Rotation of Spherical Harmonics Using Wigner Matrices Correct?

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The discussion revolves around the use of Wigner matrices for the rotation of spherical harmonics, specifically questioning the appropriateness of using the Euler angle beta in the transformation. The user expresses uncertainty about the validity of their approach since beta is an Euler angle, while theta and phi are not. It is suggested that beta will ultimately be replaced by the rotation value, resulting in a function dependent on theta and phi. Clarification on the correct application of these angles in the context of spherical harmonics is sought. The conversation emphasizes the need for proper angle representation in mathematical transformations.
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Homework Statement
Rotate the spherical harmonic $$\ket{l=2, m=1}=Y_{2, 1}$$ an angle of π/4 about the y-axis.
Relevant Equations
$$\sum_{m'=-l}^{l} {d^{(l)}}_{m, m'} Y_{l, m'}$$
I tried using the Wigner matrices:

$$\sum_{m'=-2}^{2} {d^{(2)}}_{1m'} Y_{2; m'}={d^{(2)}}_{1 -2} Y_{2; -2} + {d^{(2)}}_{1 -1} Y_{2; -1} + ...= -\frac{1-\cos(\beta)}{2} \sin(\beta) \sqrt{\frac{15}{32 \pi}} \sin^2(\theta) e^{-i \phi} + ...$$

where $$\beta=\frac{\pi}{4}$$. But I don't know if this is ok since $$\beta$$ is an Euler angle while $$\theta$$ and $$\phi$$ are not. If this is not right, what should I do?
 
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The angle ##\beta## will go away as it is replaced by the value of the rotation, leaving a function of ##(\theta,\phi)##, which is what you want.
 

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