code: def Bx(self):        
          
          #  0         self.b3  -self.b2
          # -self.b3   0         self.b1
          #  self.b2  -self.b1   0
          
          
          m = np.zeros((len(self.b1), 3, 3))
          m[:, 0, 1] = +self.b3
          m[:, 0, 2] = -self.b2
          m[:, 1, 0] = -self.b3
          m[:, 1, 2] = +self.b1
          m[:, 2, 0] = +self.b2
          m[:, 2, 1] = -self.b1
          return m
docs: Compute a stack of skew-symmetric matrices which can be multiplied
          by 'a' to get the cross product. See:
  
          http://en.wikipedia.org/wiki/Cross_product#Conversion_to_matrix_multiplication