no. R2
An L ∞ estimate and a superconvergence result for a Galerkin method for elliptic equations based on tensor products of piecewise polynomials
Revue française d'automatique, informatique, recherche opérationnelle. Analyse numérique, Tome 8 (1974) no. R2, pp. 61-66. 
@article{M2AN_1974__8_2_61_0,
     author = {Douglas, Jim Jr. and Dupont, Todd and Wheeler, Mary Fanett},
     title = {An $L^\infty $ estimate and a superconvergence result for a {Galerkin} method for elliptic equations based on tensor products of piecewise polynomials},
     journal = {Revue fran\c{c}aise d'automatique, informatique, recherche op\'erationnelle. Analyse num\'erique},
     pages = {61--66},
     publisher = {Dunod},
     address = {Paris},
     volume = {8},
     number = {R2},
     year = {1974},
     mrnumber = {359358},
     zbl = {0315.65062},
     language = {en},
     url = {http://www.numdam.org/item/M2AN_1974__8_2_61_0/}
}
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Douglas, Jim Jr.; Dupont, Todd; Wheeler, Mary Fanett. An $L^\infty $ estimate and a superconvergence result for a Galerkin method for elliptic equations based on tensor products of piecewise polynomials. Revue française d'automatique, informatique, recherche opérationnelle. Analyse numérique, Tome 8 (1974) no. R2, pp. 61-66. http://www.numdam.org/item/M2AN_1974__8_2_61_0/

[1] J. H. Bramble and J. E. Osborn, Rate of convergence estimates for nonselfadjoint eigenvalue approximations, Math. Comp., 27 (1973), 525-549. | MR | Zbl

[2] J. Jr Douglas, and T. Dupont Galerkin approximations for the two point boundary problem using continuous piecewise-polynomial spaces, Numer. Math.,, 22 (1974), 99-109. | MR | Zbl

[3] J. Jr Douglas, and T. Dupont, Superconvergence for Galerkin methods for the two point boundary problem via local projections, Numer. Math., 21 (1973), 270-278. | MR | Zbl

[4] J. Jr. Douglas, T. Dupont and L. Wahlbin, Optimal L∞ error estimates for Galerkin approximations to solutions of two point boundary problems, to appear. | Zbl

[5] J. Jr. Douglas, T. Dupont and M. F. Wheeler, A quasi-projection approximation applied to Galerkin procedures for parabolic and hyperbolic equations, to appear.

[6] J. Jr. Douglas, T. Dupont and M. F. Wheeler, A Galerkin procedure for approximating the flux on the boundary for elliptic and parabolic boundary value problems, this Journal, 47-59. | Numdam | MR | Zbl