From 2251f2943b0f0b79e3983d7769faab22a2227baf Mon Sep 17 00:00:00 2001
From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Thu, 7 Sep 2023 17:59:00 -0600
Subject: [PATCH] Add a reference to an article.

---
 doc/doxygen/references.bib                         | 12 ++++++++++++
 .../deal.II/numerics/vector_tools_constraints.h    | 14 ++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/doc/doxygen/references.bib b/doc/doxygen/references.bib
index 286b2e2594..966ff103f6 100644
--- a/doc/doxygen/references.bib
+++ b/doc/doxygen/references.bib
@@ -2206,3 +2206,15 @@
   title = {A Uniform Additive {S}chwarz Preconditioner for High-Order Discontinuous {G}alerkin Approximations of Elliptic Problems},
   journal = {Journal of Scientific Computing}
 }
+
+@article{Engelman1982,
+  author = { Engelman, M. S. and Sani, R. L. and Gresho, P. M. },
+  title = { The implementation of normal and/or tangential boundary conditions in finite element codes for incompressible fluid flow },
+  journal = { Int. J. Numer. Meth. Fluids },
+  year = { 1982 },
+  volume = { 2 },
+  issue = { 3 },
+  pages = { 225--238 },
+  doi = {10.1002/fld.1650020302},
+  url = {http://doi.org/10.1002/fld.1650020302},
+}
diff --git a/include/deal.II/numerics/vector_tools_constraints.h b/include/deal.II/numerics/vector_tools_constraints.h
index 99445c9b00..ab4cc0d6c5 100644
--- a/include/deal.II/numerics/vector_tools_constraints.h
+++ b/include/deal.II/numerics/vector_tools_constraints.h
@@ -270,6 +270,13 @@ namespace VectorTools
    * at some points on the boundary is an artifact of the way it is created,
    * it is not constrained to be zero at these points.
    *
+   *
+   * <h4>Further reading</h4>
+   *
+   * A description of some of the techniques used in this function,
+   * along with a discussion of difficulties encountered with this
+   * kind of boundary conditions can be found in @cite Engelman1982 .
+   *
    * @ingroup constraints
    *
    * @see
@@ -391,6 +398,13 @@ namespace VectorTools
    * VectorTools::compute_no_normal_flux_constraints(), and leaves the one
    * component unconstrained that is constrained by that function.
    *
+   *
+   * <h4>Further reading</h4>
+   *
+   * A description of some of the techniques used in this function,
+   * along with a discussion of difficulties encountered with this
+   * kind of boundary conditions can be found in @cite Engelman1982 .
+   *
    * @ingroup constraints
    *
    * @see
-- 
2.39.5