diff --git a/usersguide-sphinx/source/ExternalFunc1.c b/usersguide-sphinx/source/ExternalFunc1.c
new file mode 100644
index 00000000000..070ccb5e10f
--- /dev/null
+++ b/usersguide-sphinx/source/ExternalFunc1.c
@@ -0,0 +1,6 @@
+double ExternalFunc1_ext(double x)
+{
+  double res;
+  res = x+2.0*x*x;
+  return res;
+}
diff --git a/usersguide-sphinx/source/ExternalFunc1.h b/usersguide-sphinx/source/ExternalFunc1.h
new file mode 100644
index 00000000000..0ff9fe9aed4
--- /dev/null
+++ b/usersguide-sphinx/source/ExternalFunc1.h
@@ -0,0 +1 @@
+double ExternalFunc1_ext(double);
diff --git a/usersguide-sphinx/source/ExternalFunc2.c b/usersguide-sphinx/source/ExternalFunc2.c
new file mode 100644
index 00000000000..061284300b9
--- /dev/null
+++ b/usersguide-sphinx/source/ExternalFunc2.c
@@ -0,0 +1,6 @@
+double ExternalFunc2(double x)
+{
+  double res;
+  res = (x-1.0)*(x+2.0);
+  return res;
+}