Let PPP be a partition of a set of the interval [a,b][a,b][a,b]. Then arrange the elements of PPP in increasing order: a=t0<t1<⋯<tk=b.a = t_0 < t_1< \cdots< t_k = b.a=t0<t1<⋯<tk=b. The [ti,ti+1][t_i, t_i+1][ti,ti+1] are the subintervals of [a,b][a,b][a,b] given by PPP.