Theorie
The Wigner-D functions are special functions on the rotation group \(SO(3)\).
In terms of Matthies (ZYZ-convention) Euler angles \({\bf R} = {\bf R}(\alpha,\beta,\gamma)\) the \(L_2\)-normalized Wigner-D function of degree \(n\) and orders \(k,l \in \{-n,\dots,n\}\) is defined by
\[ D_n^{k,l}({\bf R}) = \sqrt{2n+1} \, \mathrm e^{-\mathrm i k\gamma} \mathrm d_n^{k,l}(\cos\beta) \,e^{-\mathrm i l\alpha} \]
where \(d_n^{k,l}\), denote the real valued Wigner-d functions, which are defined in terms of Jacobi polynomial \(P_s^{a,b}\) by
\[ d_n^{k,l}(x) = (-1)^{\nu} \binom{2n-s}{s+a}^{\frac12} \binom{s+b}{b}^{-\frac12} \left(\frac{1-x}{2}\right)^{\frac{a}{2}} \left(\frac{1+x}{2}\right)^{\frac{b}2} P_s^{a,b}(x)\]
using the constants \(a =|k-l|\), \(b =|k+l|\), \(s = n - \max\{|k|,|l|\}\) and \(\nu = \min\{0,k\}+\min\{0,l\}\) if \(l \geq k\); \(\nu = \min\{0,k\}+\min\{0,l\} + k+l\) otherwise.
This definition is slightly different to other well known definitions, because the Wigner-D functions are defined compatible to the spherical harmonics which form an orthonormal basis on the 2-sphere.
In MTEX the Wigner-D and Wigner-d functions are available through the command Wigner_D
% the Wigner-d function of degree 1
beta = 0.5;
d = Wigner_D(1,beta)
% the Wigner-D function of degree 1
R = rotation.rand;
D = sqrt(3) * Wigner_D(1,R)d =
0.9388 -0.3390 -0.0612
0.3390 0.8776 0.3390
-0.0612 -0.3390 0.9388
D =
-0.5870 - 0.7635i -1.1906 - 0.2521i 0.6755 - 0.3674i
-0.9255 - 0.7903i 0.1941 + 0.0000i -0.9255 + 0.7903i
0.6755 + 0.3674i -1.1906 + 0.2521i -0.5870 + 0.7635iHere the orders \(k\), \(l\) work as row and column indices.
Series Expansion
The Wigner-D functions form an orthonormal basis in \(L_2(SO(3))\). Hence, we can describe functions on the rotation group \(SO(3)\) by there harmonic representation using the class SO3FunHarmonic.
Hence we define the Wigner-D function \(D_1^{1,-1}\) by
D = SO3FunHarmonic([0;0;0;1])
D.eval(R)D = SO3FunHarmonic (xyz → xyz)
antipodal: true
isReal: false
bandwidth: 1
weight: 0
ans =
0.6755 + 0.3674iVarious normalization for the Wigner-D functions are common in the literature.
Here we define the \(L_2\)-norm by
\[ \| f \|_2 = \left(\frac1{8\pi^2}\,\int_{SO(3)} \lvert f( {\bf R}) \rvert^2 \,\mathrm d {\bf R} \right)^{1/2} \]
such that the norm of the constant function \(f=1\) is \(1\). Take a look on the section Integration of SO3Fun's.
Using that definition the Wigner-D functions in MTEX are normalized, i.e. \(\| D_n^{k,l} \|_2 = 1\) for all \(n,k,l\).
norm(D)ans =
1Some important formulas for Wigner-D functions
The Wigner-D functions are the matrix elements of the representations \(D_n \colon SO(3) \to \mathbb C^{(2n+1)\times(2n+1)}\) on \(SO(3)\). Since representations are group homomorphisms, we have \(D_n( {\bf R} \, {\bf Q} ) = \frac1{\sqrt{2n+1}} \, D_n( {\bf Q} ) \, D_n( {\bf R} ).\) Hence we get
\[ D_n^{k,l}( {\bf R} \, {\bf Q} ) = \frac1{2n+1} \sum_{j=-n}^n D_n^{k,j}( {\bf Q} )\,D_n^{j,l}( {\bf R} ). \]
Some symmetry properties of Wigner-D functions yields
\[ D_n^{k,l}( {\bf R} ) = \overline{D_n^{l,k}( {\bf R}^{-1} )}. \]
Symmetry properties of Wigner-d functions
The Wigner-d functions by construction fulfill a lot of symmetry properties. Some important are
\[ d_n^{k,l}(x) = d_n^{-k,-l}(x) = (-1)^{k+l}\, d_n^{l,k}(x) = (-1)^{k+l}\, d_n^{-l,-k}(x)\]
\[ d_n^{k,l}(x) = (-1)^{n+k+l}\,d_n^{-k,l}(-x) = (-1)^{n+k+l}\,d_n^{k,-l}(-x) \]
\[d_n^{k,l}(\cos\beta) = (-1)^{k+l}\,d_n^{k,l}(\cos(-\beta))\]
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