Carsten Carstensen, Joscha Gedicke & Donsub Rim. (1970). Explicit Error Estimates for Courant, Crouzeix-Raviart and Raviart-Thomas Finite Element Methods.
            <i>Journal of Computational Mathematics</i>. <i>30</i> (4).
            337-353.
         doi:10.4208/jcm.1108-m3677