sentences:  This is necessary because any solution of the above boundary value problem can be shifted in time by an arbitrary amount (time does not appear in the defining equations—the dynamical system is called autonomous).There are several choices for the phase constraint.If formula_21 is a known periodic orbit at a parameter value formula_22 near formula_2, then, Poincaré used
  
  which states that formula_3 lies in a plane which is orthogonal to the tangent vector of the closed curve.