Vidal, Y., A. . Huerta, and P. . Villon. “Locking in the Incompressible Limit: Pseudo-Divergence-Free Element Free Galerkin”. European Journal of Computational Mechanics, vol. 11, no. 7-8, Apr. 2002, pp. 869-92, https://journals.riverpublishers.com/index.php/EJCM/article/view/2519.