Class Sigmoid

                                  object --+        
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base.parameterizedobject.ParameterizedObject --+    
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                  base.functionfamilies.OutputFn --+
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                                                  Sigmoid

Sigmoidal (logistic) output function: 1/(1+exp-(r*x+k)).

As defined in Jochen Triesch, ICANN 2005, LNCS 3696 pp. 65-70. The parameters control the growth rate (r) and the x position (k) of the exponential.

This function is a special case of the GeneralizedLogistic function, with parameters r=r, l=0, u=1, m=-k/2r, and b=1. See Richards, F.J. (1959), A flexible growth function for empirical use. J. Experimental Botany 10: 290--300, 1959. http://en.wikipedia.org/wiki/Generalised_logistic_curve