Product of two sigmoidal membership functions
y = psigmf(x,[a1 c1 a2 c2])
The sigmoid curve plotted for the vector x depends on two parameters a and c as given by
psigmf is simply the product of two such curves plotted for the values of the vector x
|f1(x; a1, c1) × f2(x; a2, c2)|
The parameters are listed in the order [a1 c1 a2 c2].