code: def Ax(self):
          
          #  0         -self.a3   self.a2
          #  self.a3    0        -self.a1
          # -self.a2    self.a1   0        
          m = np.zeros((len(self.a1), 3, 3))
          m[:, 0, 1] = -self.a3
          m[:, 0, 2] = +self.a2
          m[:, 1, 0] = +self.a3
          m[:, 1, 2] = -self.a1
          m[:, 2, 0] = -self.a2
          m[:, 2, 1] = +self.a1        
          return m
docs: Compute a stack of skew-symmetric matrices which can be multiplied
          by 'b' to get the cross product. See:
  
          http://en.wikipedia.org/wiki/Cross_product#Conversion_to_matrix_multiplication