--- a/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/Exp.java	Wed May 30 17:59:19 2012 +0000
+++ b/flys-artifacts/src/main/java/de/intevation/flys/artifacts/math/fitting/Exp.java	Thu May 31 16:32:03 2012 +0000
@@ -3,6 +3,25 @@
 public class Exp
 extends      Function
 {
+    public static final Derivative DERIVATIVE =
+        new Derivative("W'(Q) = a^Q*log(a)*m") {
+
+        @Override
+        public de.intevation.flys.artifacts.math.Function
+            instantiate(double [] parameters)
+        {
+            final double m = parameters[0];
+            final double a = parameters[1];
+
+            return new de.intevation.flys.artifacts.math.Function() {
+                @Override
+                public double value(double Q) {
+                    return Math.pow(a, Q)*Math.log(a)*m;
+                }
+            };
+        }
+    };
+
     public Exp() {
         super(
             "exp",
@@ -23,5 +42,10 @@
             1d
         };
     }
+
+    @Override
+    public Derivative getDerivative() {
+        return DERIVATIVE;
+    }
 }
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