Quantitative uniqueness for zero-order perturbations of generalized   Baouendi-Grushin operators
  Based on a variant of the frequency function approach of Almgren, we establish an optimal bound on the vanishing order of solutions to stationary Schr\"odinger equations associated to a class of subelliptic equations with variable coefficients whose model is the so-called Baouendi-Grushin operator. Such bound provides a quantitative form of strong unique continuation that can be thought of as an analogue of the recent results of Bakri and Zhu for the standard Laplacian. 
Agnid Banerjee, Nicola Garofalo
math.AP