Function Description
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	Name: SIGMOID
	Function: f(x) =  a + b / (1 + exp(-(x-c)/d)) 
	Parameters: a b c d

Notes
*****

This function will converges fairly easily with reasonable starting values
for the parameters.

C-code for the function
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	 answer =  a + b / (1 + exp(-(x - c) / d));