Add a paragraph.

git-svn-id: https://svn.dealii.org/trunk@24402 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-31/doc/intro.dox b/deal.II/examples/step-31/doc/intro.dox
index 423e6bde2ba78211945c78c9a9a782298835831a..4d87b0865c0240ea03f52fd18b99321a39039e88 100644 (file)
--- a/deal.II/examples/step-31/doc/intro.dox
+++ b/deal.II/examples/step-31/doc/intro.dox
@@ -969,6 +969,16 @@ operator part of the momentum equation. The ability to solve problems in
 %parallel will be explored in step-32, using the same problem as
 discussed here.
 
+PETSc, which we have used in step-17 and step-18, is certainly a powerful
+library, providing a large number of functions that deal with matrices,
+vectors, and iterative solvers and preconditioners, along with lots of other
+stuff, most of which runs quite well in %parallel. It is, however, a few years
+old already than Trilinos, written in C, and generally not quite as easy to
+use as some other libraries. As a consequence, deal.II has also acquired
+interfaces to Trilinos, which shares a lot of the same functionality with
+PETSc. It is, however, a project that is several years younger, is written in
+C++ and by people who generally have put a significant emphasis on software
+design.
 
 
 <h3>The testcase</h3>