Top panel: illustration of the behaviour of the function , where N has been rescaled by a factor 10^-5 so as to simplify the representation. The dots correspond to computed values thanks to Fig. 5, while the line corresponds to a linear fit, which takes the form . The coefficients have been uniformly renormalised so as to clarify this representation. Bottom panel: similar representation for the behaviour of the function , whose linear fit takes the form . Similarly, the coefficients have been uniformly renormalised so as to clarify this representation.