Add some references

These papers show that Q(k) x Q(k - 1) is LBB-stable but that its
stability constant degrades with aspect ratio, hence superiority of Q(k)
* Q(k -2) whose stability constant only degrades with k.

tensor-product-stokes.bib

@@ -132,3 +132,26 @@ @article{isaac2015solution
   year={2015},
   publisher={SIAM}
 }
+
+
+@article{ainsworth2000stability,
+  title={The stability of mixed hp-finite element methods for {Stokes} flow on high aspect ratio elements},
+  author={Ainsworth, Mark and Coggins, Patrick},
+  journal={SIAM Journal on Numerical Analysis},
+  volume={38},
+  number={5},
+  pages={1721--1761},
+  year={2000},
+  publisher={SIAM}
+}
+
+
+@article{toselli2003mixed,
+  title={Mixed hp-finite element approximations on geometric edge and boundary layer meshes in three dimensions},
+  author={Toselli, Andrea and Schwab, Christoph},
+  journal={Numerische Mathematik},
+  volume={94},
+  pages={771--801},
+  year={2003},
+  publisher={Springer}
+}