A sigmoidal selectivity function with 50% selectivity at
weight sigmoidal_weight \(=w_{\text{sigmoid}}\) and width sigmoidal_sigma \(=\sigma\).
$$S(w) = \left(1 + \left(\frac{w}{w_{\text{sigmoid}}}\right)^{-\sigma}\right)^{-1}$$
See also
gear_params() for setting the selectivity parameters.
Other selectivity functions:
double_sigmoid_length(),
knife_edge(),
sigmoid_length()
