\[ \dot{\hat{x}} = f(\hat{x}, u) + L \text{sgn}(x - \hat{x}) \] \[ u = -k_1 (\hat{x} - x_{ref}) - k_2 \dot{\hat{x}} \] \[ e = x - \hat{x}, \quad \dot{e} = f(x, u) - \dot{\hat{x}} \]