A note on geometric conditions for boundary control of wave equations with variable coefficients ✩

The analytical condition given by Wyler for boundary stabilization of wave equations with variable coefficients is compared with the geometrical condition derived by Yao in terms of the Riemannian geometry method for exact controllability of wave equations with variable coefficients. It is shown that these two conditions are equivalent.  2002 Elsevier Science (USA). All rights reserved.