--- a/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/InvPow.java	Tue Jun 26 10:45:33 2012 +0000
+++ b/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/InvPow.java	Tue Jun 26 10:54:12 2012 +0000
@@ -3,22 +3,59 @@
 public class InvPow
 extends      Function
 {
+    public static final Derivative DERIVATIVE =
+        new Derivative("Q'(W) = ((W-d)/a)^(1/c)/(c*(W-d))") {
+
+        @Override
+        public de.intevation.flys.artifacts.math.Function
+            instantiate(double [] parameters)
+        {
+                  double  a = parameters[0];
+            final double  c = parameters[1];
+            final double  d = parameters[2];
+            final double _1a = 1d/a;
+            final double _1c = 1d/c;
+
+            return new de.intevation.flys.artifacts.math.Function() {
+                @Override
+                public double value(double W) {
+                    double Wd = W-d;
+                    return Math.pow(Wd*_1a, _1c)/(c*Wd);
+                }
+            };
+        }
+    };
+
     public static final Function INSTANCE = new InvPow();
 
     public InvPow() {
-        // TODO: Implement me!
+        super(
+            "pow",
+            "Q(W) = ((W-d)/a)^(1/c)",
+            new String [] { "a", "c", "d" });
     }
 
     @Override
     public double value(double W, double [] parameters) {
-        // TODO: Implement me!
-        return 0d;
+        double a = parameters[0];
+        double c = parameters[1];
+        double d = parameters[2];
+        return Math.pow((W-d)/a, 1d/c);
     }
 
     @Override
-    public double [] gradient(double Q, double [] parameters) {
-        // TODO: Implement me!
-        return null;
+    public double [] gradient(double W, double [] parameters) {
+        double a = parameters[0];
+        double c = parameters[1];
+        double d = parameters[2];
+        double _1c = 1d/c;
+        double Wdac = Math.pow((W-d)/a, 1d/c);
+        double Wd = W-d;
+        return new double [] {
+            -Wdac/(a*c),
+            (Wdac*Math.log(Wd/a))/(c*c),
+            -Wdac/(c*Wd)
+        };
     }
 
     @Override