[chronojump] Fixed LeastSquares formula

[Xavier Padullés <xpadulles src gnome org>]

commit 60f49135ad46531c013acb42dd2cc4f217829562
Author: Xavier Padullés <x padulles gmail com>
Date:   Mon Sep 19 14:21:47 2016 +0200

    Fixed LeastSquares formula

 src/utilMath.cs |   12 +++++++-----
 1 files changed, 7 insertions(+), 5 deletions(-)
---
diff --git a/src/utilMath.cs b/src/utilMath.cs
index 8bf4bb9..839afb2 100644
--- a/src/utilMath.cs
+++ b/src/utilMath.cs
@@ -75,21 +75,23 @@ public class LeastSquares
                double sumX = 0; //sumatory of the X values
                double sumX2 = 0; //sumatory of the squared X values
                double sumX3 = 0; //sumatory of the cubic X values
+               double sumX4 = 0; //sumatory of the forth power of X values
 
                for(int i = 0; i < numMeasures; i++){
                        sumX = sumX + measures[i].X;
                        sumX2 = sumX2 + measures[i].X * measures[i].X;
                        sumX3 = sumX3 + measures[i].X * measures[i].X * measures[i].X;
+                       sumX4 = sumX3 + measures[i].X * measures[i].X * measures[i].X * measures[i].X;
                }
 
-               double detA = numMeasures*sumX2*sumX3 + 2*sumX*sumX2*sumX3 - sumX2*sumX2*sumX2 - 
sumX2*sumX2*sumX3 - numMeasures*sumX3*sumX3;
+               double detA = numMeasures*sumX2*sumX4 + 2*sumX*sumX2*sumX3- sumX2*sumX2*sumX2 - 
sumX*sumX*sumX4 - numMeasures*sumX3*sumX3;
                if(detA != 0){
                        double [,] invA = new double[3,3];
 
-                       invA[0,0] = ( sumX2*sumX3 - sumX3*sumX3) / detA;
-                       invA[0,1] = (-sumX*sumX3  + sumX2*sumX3) / detA;
+                       invA[0,0] = ( sumX2*sumX4 - sumX3*sumX3) / detA;
+                       invA[0,1] = (-sumX*sumX4 + sumX2*sumX3) / detA;
                        invA[0,2] = ( sumX*sumX3  - sumX2*sumX2) / detA;
-                       invA[1,1] = ( numMeasures*sumX3 - sumX2*sumX2) / detA;
+                       invA[1,1] = ( numMeasures*sumX4 - sumX2*sumX2) / detA;
                        invA[1,2] = (-numMeasures*sumX3 + sumX*sumX2 ) / detA;
                        invA[2,2] = ( numMeasures*sumX2 - sumX*sumX  ) / detA;
 
@@ -107,7 +109,7 @@ public class LeastSquares
                        Success = true;
                        Coef = coef;
                } else {
-                       LogB.Error("Invalid values");
+                       LogB.Error("Determinant of matrix equal to zero");
                        Success = false;
                }
        }