Sigmoidal membership function
y = sigmf(x,[a c])
The sigmoidal function, sigmf(x,[a c]), as given in the following equation by f(x,a,c) is a mapping on a vector x, and depends on two parameters a and c.
Depending on the sign of the parameter a, the sigmoidal membership function is inherently open to the right or to the left, and thus is appropriate for representing concepts such as "very large" or "very negative." More conventional-looking membership functions can be built by taking either the product or difference of two different sigmoidal membership functions. For more information seedsigmf and psigmf.