docs: def K(self, r, R, kwargs):
          """
          equation A16 im Mamon & Lokas
          :param r: 3d radius
          :param R: projected 2d radius
          :return:
          """
          u = r / R
          if np.min(u) < 1:
              raise ValueError("3d radius is smaller than projected radius! Does not make sense.")
  
          if self._type == 'const_wrong':
              beta = kwargs['beta']
              k = 1./2. * u**(2*beta - 1.) * ((3./2 - beta) * np.sqrt(np.pi) * special.gamma(beta - 1./2)/special.gamma(beta)
                          + beta * self._B(x=1./u**2, a=beta+1./2, b=1./2) - self._B(x=1./u**2, a=beta-1./2, b=1./2))
          elif self._type == 'const':
              beta = kwargs['beta']
              k = np.sqrt(1 - 1./u**2) / (1. - 2*beta) + np.sqrt(np.pi)/2 * special.gamma(beta - 1./2)/special.gamma(beta)\
                  * (3./2 - beta) * u**(2*beta - 1.) * (1 - special.betainc(beta+1./2, 1./2, 1./u**2))
          elif self._type == 'isotropic':
              k = np.sqrt(1 - 1./u**2)
          elif self._type == 'radial':
              k = np.pi/4 * u - 1./2 * np.sqrt(1 - 1./u**2) - u/2. * np.arcsin(1./u)
          elif self._type == 'OsipkovMerritt':
              r_ani = kwargs['r_ani']
              ua = r_ani / R
              k = (ua**2 + 1./2) / (ua**2 + 1)**(3./2) * (u**2 + ua**2) / u * np.arctan(np.sqrt((u**2 - 1) / (ua**2 + 1))) \
                  - 1./2/(ua**2 + 1) * np.sqrt(1 -1./u**2)
          elif self._type == 'Colin':
              r_ani = kwargs['r_ani']
              ua = r_ani / R
              if ua == 1:
                  k = (1 + 1./u) * np.arccosh(u) - 1./6 * (8./u + 7) * np.sqrt((u-1.)/(u+1.))
              else:
                  k = 0.5 / (ua**2 - 1) * np.sqrt(1 - 1./u**2) + (1. + ua/u) * np.cosh(u) - np.sign(ua - 1) * ua * \
                              (ua**2 - 0.5) / np.abs(ua**2-1)**(3./2) * (1. + ua/u) * np.arccosh((ua*u + 1)/(u + ua))
          else:
              raise ValueError('anisotropy type %s not supported!' % self._type)
          return k