The Target Discounted-Sum Problem

The target discounted-sum problem is the following: Given a rational discount factor 0 <; λ <; 1 and three rational values a, b, and t, does there exist a finite or an infinite sequence w ∈ {a, b}* or w ∈ {a, b}^ω, such that Σ_i=0^|w| w(i)λⁱ equals t? The problem turns out to relate to many fields of mathematics and computer science, and its decidability question is surprisingly hard to solve. We solve the finite version of the problem, and show the hardness of the infinite version, linking it to various areas and open problems in mathematics and computer science: β-expansions, discounted-sum automata, piecewise affine maps, and generalizations of the Cantor set. We provide some partial results to the infinite version, among which are solutions to its restriction to eventually-periodic sequences and to the cases that λ ≥ 1/2 or λ =1/n, for every n ∈ N. We use our results for solving some open problems on discounted-sum automata, among which are the exact-value problem for nondeterministic automata over finite words and the universality and inclusion problems for functional automata.