--- a/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/InvSQPow.java	Tue Jun 26 10:45:33 2012 +0000
+++ b/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/InvSQPow.java	Tue Jun 26 10:54:12 2012 +0000
@@ -3,28 +3,57 @@
 public class InvSQPow
 extends      Function
 {
+    public static final Derivative DERIVATIVE =
+        new Derivative("Q'(S) = (S/a)^(1/b)/(b*S)") {
+
+        @Override
+        public de.intevation.flys.artifacts.math.Function
+            instantiate(double [] parameters)
+        {
+            final double _1a = 1d/parameters[0];
+            final double   b = parameters[1];
+            final double _1b = 1d/b;
+
+            return new de.intevation.flys.artifacts.math.Function() {
+                @Override
+                public double value(double S) {
+                    return Math.pow(S*_1a, _1b)/(b*S);
+                }
+            };
+        }
+    };
     public static final Function INSTANCE = new InvSQPow();
 
     public InvSQPow() {
-        // TODO: Implement me!
+        super(
+            "inv-sq-pow",
+            "Q(S) = Q=(S/a)^(1/b)",
+            new String [] { "a", "b" });
     }
 
     @Override
-    public double value(double W, double [] parameters) {
-        // TODO: Implement me!
-        return 0d;
+    public double value(double S, double [] parameters) {
+        double a = parameters[0];
+        double b = parameters[1];
+        return Math.pow(S/a, 1d/b);
     }
 
     @Override
-    public double [] gradient(double Q, double [] parameters) {
-        // TODO: Implement me!
-        return null;
+    public double [] gradient(double S, double [] parameters) {
+        double a     = parameters[0];
+        double b     = parameters[1];
+        double Sa    = S/a;
+        double _1b   = 1d/b;
+        double eSa1b = Math.pow(Sa, _1b);
+        return new double [] {
+            -eSa1b/(a*b),
+            -(eSa1b*Math.log(Sa))/(b*b)
+        };
     }
 
     @Override
     public Derivative getDerivative() {
-        // TODO: Implement me!
-        return null;
+        return DERIVATIVE;
     }
 
     @Override