f(x) \; = \;  \frac{ x^2-a }{x^2-b }

a = -1.3 (slider from -5 to 5, step 0.1)
b = 1.6 (slider from -5 to 5, step 0.1)
f(x) = (x^(2.0) - a) / (x^(2.0) - b)