Recently, Constantinescu and Ilie proved a variant of the well-known periodicity theoremof Fine and Wilf in the case of two relatively prime abelian periods and conjectured aresult for the case of two non-relatively prime abelian periods. In this paper, we answersome open problems they suggested. We show that their conjecture is false but we givebounds, that depend on the two abelian periods, such that the conjecture is true for allwords having length at least those bounds and show that some of them are optimal. We alsoextend their study to the context of partial words, giving optimal lengths and describingan algorithm for constructing optimal words.

We provide an algorithm for listing all minimal 2-dominating sets of a tree of ordern in time 𝒪(1.3248n). This implies that every tree has at most1.3248n minimal 2-dominating sets. We also show that thisbound is tight.

We discuss how much space is sufficient to decide whether a unary given numbern is a prime. We show thatO(log log n) space is sufficient for a deterministicTuring machine, if it is equipped with an additional pebble movable along the input tape,and also for an alternating machine, if the space restriction applies only to itsaccepting computation subtrees. In other words, the language is a prime is inpebble–DSPACE(log log n) and also inaccept–ASPACE(log log n). Moreover, if the givenn is composite, such machines are able to find a divisor ofn. Since O(log log n) space is toosmall to write down a divisor, which might requireΩ(log n) bits, the witness divisor is indicated by theinput head position at the moment when the machine halts.

We give several new applications of the wreath product of forest algebras to the study oflogics on trees. These include new simplified proofs of necessary conditions fordefinability in CTL and first-order logic with the ancestor relation; asequence of identities satisfied by all forest languages definable inPDL; and new examples of languages outside CTL, alongwith an application to the question of what properties are definable in bothCTL and LTL.

The traveling salesman problem (TSP) is one of the most fundamental optimizationproblems. We consider the β-metric traveling salesman problem(Δβ-TSP), i.e., the TSPrestricted to graphs satisfying the β-triangle inequalityc({v,w}) ≤ β(c({v,u}) + c({u,w})),for some cost function c and any three vertices u,v,w.The well-known path matching Christofides algorithm (PMCA) guarantees an approximationratio of 3β2/2 and is the best known algorithm for theΔβ-TSP, for 1 ≤ β ≤ 2. Weprovide a complete analysis of the algorithm. First, we correct an error in the originalimplementation that may produce an invalid solution. Using a worst-case example, we thenshow that the algorithm cannot guarantee a better approximation ratio. The example canalso be used for the PMCA variants for the Hamiltonian path problem with zero and oneprespecified endpoints. For two prespecified endpoints, we cannot reuse the example, butwe construct another worst-case example to show the optimality of the analysis also inthis case.