'Wigner small-d function.
Returns an instance of the WignerD class corresponding to the Wigner-D
function specified by the parameters with the alpha and gamma angles
given as 0.
Parameters
j : Number
Total angular momentum
m : Number
Eigenvalue of angular momentum along axis after rotation
mp : Number
Eigenvalue of angular momentum along rotated axis
beta : Number, Symbol
Second Euler angle of rotation
Examples
Return the Wigner-D matrix element for a defined rotation, both
numerical and symbolic:
>>> from sympy.physics.quantum.spin import Rotation
>>> from sympy import pi, symbols
>>> beta = symbols(\'beta\')
>>> Rotation.d(1, 1, 0, pi/2)
WignerD(1, 1, 0, 0, pi/2, 0)
See Also
WignerD: Symbolic Wigner-D function'

@classmethod
def d(cls, j, m, mp, beta):

    return WignerD(j, m, mp, 0, beta, 0)
