From Sturmian and Christoffel words we derive a strictly increasing function ∆ : [0, ∞) → R. This function is continuous at every irrational point, while at rational points, left-continuous but not right-continuous. Moreover, it assumes algebraic integers at rationals, and transcendental numbers at irrationals. We also see that the differentiation of ∆ distinguishes some irrationality measures of real numbers.
IntroductionFor any real number α ∈ [0, 1], we consider the dynamics of rotationUsing an alphabet A = {a, b} with some integers 0 ≤ a < b, the itinerary (R n α (ρ)) n≥0 of some ρ is recorded according to a partitionIn other words, if ρ ∈ [0, 1] has traveled over P, then its infinite history (s α,ρ (n)) n≥0 of rotations is determined by the following rule:In case that we adopt the partition P ′ , a sequence (s ′ α,ρ (n)) n≥0 is also defined in a similar fashion. One can observe that if A = {0, 1}, then these infinite words are obtained by the formulaewhere ⌊t⌋ is the largest integer not greater than t, and ⌈t⌉ is the smallest integer not less than t. The infinite words s α,ρ , s ′ α,ρ are called lower and upper * Keywords: devil's staircase, Sturmian word, Christoffel word, irrationality measure, Liouville number.