S_∞ : Proportion d'individus susceptibles 
n'ayant pas été contaminés lors de l'épidémie

g = Function(x, 0, 1)
R_0 = 2.89 (slider from 1 to 10, step 0.01)
f = Function(exp(-(1 - x) * R_0), 0, 1)
A, B = Intersect(f, g, 0, 0.99)
a = OrthogonalLine(A, xAxis)
S_∞ = Intersect(a, xAxis)
b = Segment(A, S_∞)
O = Intersect(g, xAxis, 1)
DistanceOS_∞ = Distance(O, S_∞)
texte3: "S_∞= " + DistanceOS_∞ + ""