* calc/calc-mtx.el (math-lud-pivot-check): New function.

(math-do-matrix-lud): Use `math-lud-pivot-check' to check the size
of potential pivots.

2 files changed, 21 insertions, 1 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index b2e5e98016e..7e5d94f9703 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,9 @@
+2011-02-01  Jay Belanger  <jay.p.belanger@gmail.com>
+
+        * calc/calc-mtx.el (math-lud-pivot-check): New function.
+        (math-do-matrix-lud): Use `math-lud-pivot-check' to check the size
+        of potential pivots.
+
 2011-01-31  Alan Mackenzie  <acm@muc.de>
 
         * progmodes/cc-cmds.el (c-forward-over-illiterals): Continue
diff --git a/lisp/calc/calc-mtx.el b/lisp/calc/calc-mtx.el
index 9941c11ff14..5ec15005b48 100644
--- a/lisp/calc/calc-mtx.el
+++ b/lisp/calc/calc-mtx.el
@@ -232,6 +232,20 @@
           (setq math-lud-cache (cons (cons m entry) math-lud-cache)))
         lud))))
 
+
+(defun math-lud-pivot-check (a)
+  "Determine a useful value for checking the size of potential pivots
+in LUD decomposition."
+  (cond ((eq (car-safe a) 'mod)
+         (if (and (math-integerp (nth 1 a))
+                  (math-integerp (nth 2 a))
+                  (eq (math-gcd (nth 1 a) (nth 2 a)) 1))
+             1
+           0))
+        (t
+         (math-abs-approx a))))
+
+
 ;;; Numerical Recipes section 2.3; implicit pivoting omitted.
 (defun math-do-matrix-lud (m)
   (let* ((lu (math-copy-matrix m))
@@ -261,7 +275,7 @@
                                             (nth j (nth k lu))))
                 k (1+ k)))
         (setcar (nthcdr j (nth i lu)) sum)
-        (let ((dum (math-abs-approx sum)))
+        (let ((dum (math-lud-pivot-check sum)))
           (if (Math-lessp big dum)
               (setq big dum
                     imax i)))