def inv(self):
        """In place invert"""
        self.v = 1/self.v
        tmp = self.v**2
        if self.deriv > 1:
            self.dd[:] = tmp*(2*self.v*np.outer(self.d, self.d) - self.dd)
        if self.deriv > 0:
            self.d[:] = -tmp*self.d[:]