[gnumeric] EIGEN: fix termination condition.

[Morten Welinder <mortenw src gnome org>]

commit 86e758166f356515f24bb5d002e6a3888542ef11
Author: Morten Welinder <terra gnome org>
Date:   Wed Jan 9 11:04:04 2013 -0500

    EIGEN: fix termination condition.

 ChangeLog      |    5 +++++
 NEWS           |    1 +
 src/mathfunc.c |    9 +++++++--
 3 files changed, 13 insertions(+), 2 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 723fb81..673e32b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2013-01-09  Morten Welinder  <terra gnome org>
+
+	* src/mathfunc.c (gnm_matrix_eigen): Silence warning.  Document a
+	bit.
+
 2013-01-07  Morten Welinder  <terra gnome org>
 
 	* src/mathfunc.c (gnm_matrix_eigen): Detect underflow of the
diff --git a/NEWS b/NEWS
index 4fa55ef..d10787e 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,7 @@ Morten:
 	* Start working on warnings from newer gcc.
 	* Avoid using some deprecated symbols.
 	* Fix multihead issue with clipboard.
+	* Improve EIGEN.   [#691378]
 
 --------------------------------------------------------------------------
 Gnumeric 1.12.0
diff --git a/src/mathfunc.c b/src/mathfunc.c
index 4df86aa..75ff768 100644
--- a/src/mathfunc.c
+++ b/src/mathfunc.c
@@ -6540,7 +6540,12 @@ gnm_matrix_eigen_update (guint k, gnm_float t, gnm_float *eigenvalues, gboolean
 	}
 }
 
-/* Calculates the eigenvalues and eigenvectors of a real symmetric matrix.
+/*
+ * Calculates the eigenvalues and eigenvectors of a real symmetric matrix.
+ *
+ * This is the Jacobi iterative process in which we use a sequence of
+ * Jacobi rotations (two-sided Givens rotations) in order to reduce the
+ * magnitude of off-diagonal elements while preserving eigenvalues.
  */
 gboolean
 gnm_matrix_eigen (gnm_float **matrix, gnm_float **eigenvectors, gnm_float *eigenvalues, int size)
@@ -6586,7 +6591,7 @@ gnm_matrix_eigen (gnm_float **matrix, gnm_float **eigenvectors, gnm_float *eigen
 		pivot = matrix[m][l];
 		/* pivot is (m,l) */
 		if (pivot == 0) {
-			g_printerr ("gnm_matrix_eigen underflow in pivot.\n");
+			/* All remaining off-diagonal elements are zero.  We're done.  */
 			break;
 		}