From: Rene Gassmoeller <R.Gassmoeller@mailbox.org>
Date: Sun, 8 Nov 2015 06:06:47 +0000 (-0600)
Subject: Fix MappingQ1
X-Git-Tag: v8.4.0-rc2~241^2
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fpull%2F1846%2Fhead;p=dealii.git

Fix MappingQ1
---

diff --git a/source/fe/mapping_q_generic.cc b/source/fe/mapping_q_generic.cc
index 9f5eec7b3d..4696483c38 100644
--- a/source/fe/mapping_q_generic.cc
+++ b/source/fe/mapping_q_generic.cc
@@ -106,8 +106,10 @@ namespace internal
             eta1 = -c/b;
             eta2 = -c/b;
           }
-        // special case #2: if c is very small:
-        else if (std::abs(c/b) < 1e-12)
+        // special case #2: if c is very small or the square root of the
+        // discriminant is nearly b.
+        else if (std::abs(c) < 1e-12*std::abs(b)
+                 || std::abs(std::sqrt(discriminant) - b) <= 1e-14*std::abs(b))
           {
             eta1 = (-b - std::sqrt(discriminant)) / (2*a);
             eta2 = (-b + std::sqrt(discriminant)) / (2*a);