(i) a tangent at [𝑆] /𝐾 = 0 whose equation is: /𝐾 for several 𝜎 values labelled on each curve (identical to those in Fig. ). The red dotted lines are the limiting cases given by (18a) and (18b) for 𝜎 = 4. The black line (𝜎 = 0) is a line with a unit slope.