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