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+/////////////////////////////////////////////////////////////////////////////
+//
+// NNActivation.h
+//
+//   all the activation functions of the neurons
+//
+//   header file
+//
+//   Copyright (c) 2005 Georg Holzmann <grh@gmx.at>
+//
+//   This program is free software; you can redistribute it and/or
+//   modify it under the terms of the GNU General Public License
+//   as published by the Free Software Foundation; either version 2
+//   of the License, or (at your option) any later version.
+//
+/////////////////////////////////////////////////////////////////////////////
+
+
+#ifndef _INCLUDE_ACTIVATION_NET__
+#define _INCLUDE_ACTIVATION_NET__
+
+
+#include <math.h>
+
+namespace TheBrain
+{
+
+//------------------------------------------------------
+/* implementation of the different activation functions
+ * and it's derivations
+ */
+
+/* Linear activation function.
+ * span: -inf < y < inf
+ * y = x
+*/
+#define LINEAR 0
+
+/* Sigmoid activation function.
+ * span: 0 < y < 1
+ * y = 1/(1 + exp(-x)), y' = y*(1 - y)
+ */
+#define SIGMOID 1
+
+/* Symmetric sigmoid activation function, aka. tanh.
+ * span: -1 < y < 1
+ * y = tanh(x) = 2/(1 + exp(-2*x)) - 1, d = 1-(y*y)
+*/
+#define TANH 2
+
+// linear function
+float act_linear(float value)
+{ return value; }
+
+// derivation of the linear function
+float act_linear_derive(float value)
+{ return 1; }
+
+// sigmoid function
+float act_sigmoid(float value)
+{ return (1.0f/(1.0f + exp(-value))); }
+
+// derivation of the sigmoid function
+float act_sigmoid_derive(float value)
+{ return (value * (1.0f - value)); }
+
+// tanh function
+float act_tanh(float value)
+{ return (2.0f/(1.0f + exp(-2.0f * value)) - 1.0f); }
+
+// derivation of the tanh function
+float act_tanh_derive(float value)
+{ return (1.0f - (value*value)); }
+
+
+} // end of namespace 
+
+#endif // _INCLUDE_ACTIVATION_NET__