--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/artifacts/src/main/java/org/dive4elements/river/artifacts/math/fitting/InvQuad.java	Thu Apr 25 15:23:37 2013 +0200
@@ -0,0 +1,69 @@
+package org.dive4elements.river.artifacts.math.fitting;
+
+public class InvQuad
+extends      Function
+{
+    public static final Derivative DERIVATIVE =
+        new Derivative("Q'(W) = 1/sqrt(4*n*(W-b)+m^2)") {
+
+        @Override
+        public org.dive4elements.river.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 org.dive4elements.river.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() {
+        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) {
+        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 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() {
+        return DERIVATIVE;
+    }
+
+    @Override
+    public Function getInverse() {
+        return Quad.INSTANCE;
+    }
+}
+// vim:set ts=4 sw=4 si et sta sts=4 fenc=utf8 :