--- a/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/InvQuad.java	Tue Jun 26 10:45:33 2012 +0000
+++ b/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/InvQuad.java	Tue Jun 26 10:54:12 2012 +0000
@@ -3,28 +3,62 @@
 public class InvQuad
 extends      Function
 {
+    public static final Derivative DERIVATIVE =
+        new Derivative("Q'(W) = 1/sqrt(4*n*(W-b)+m^2)") {
+
+        @Override
+        public de.intevation.flys.artifacts.math.Function
+            instantiate(double [] parameters)
+        {
+                  double n  = parameters[0];
+                  double m  = parameters[1];
+            final double b  = parameters[2];
+            final double n4 = 4d*n;
+            final double mm = m*m;
+
+            return new de.intevation.flys.artifacts.math.Function() {
+                @Override
+                public double value(double W) {
+                    return 1d/Math.sqrt(n4*(W-b)+mm);
+                }
+            };
+        }
+    };
+
     public static final Function INSTANCE = new InvQuad();
 
     public InvQuad() {
-        // TODO: Implement me!
+        super(
+            "inv-quad",
+            "(sqrt(4*n*W-4*b*n+m^2)-m)/(2*n)",
+            new String [] { "n", "m", "b" });
     }
 
     @Override
     public double value(double W, double [] parameters) {
-        // TODO: Implement me!
-        return 0d;
+        double n = parameters[0];
+        double m = parameters[1];
+        double b = parameters[2];
+        return (Math.sqrt(4d*n*(W - b) + m*m)-m)/(2d*n);
     }
 
     @Override
-    public double [] gradient(double Q, double [] parameters) {
-        // TODO: Implement me!
-        return null;
+    public double [] gradient(double W, double [] parameters) {
+        double n  = parameters[0];
+        double m  = parameters[1];
+        double b  = parameters[2];
+        double Wb = W-b;
+        double sn4Wb = Math.sqrt(4d*n*Wb + m*m);
+        return new double [] {
+            Wb/(n*sn4Wb)-(sn4Wb-m)/(2d*n*n),
+            (m/sn4Wb-1d)/(2d*n),
+            -1d/sn4Wb
+        };
     }
 
     @Override
     public Derivative getDerivative() {
-        // TODO: Implement me!
-        return null;
+        return DERIVATIVE;
     }
 
     @Override