Hua Chen, Hongge Chen, Junfang Wang & Nana Zhang. (2020). Lower Bounds of Dirichlet Eigenvalues for General Grushin Type Bi-Subelliptic Operators.
            <i>Analysis in Theory and Applications</i>. <i>35</i> (1).
            66-84.
         doi:10.4208/ata.OA-0002