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Given an edge-weighted undirected graph G = (V , E , w) and a subset T ⊆ V of p terminals, a k-Steiner forest spanning all the terminals in T includes k branches, where every branch is a Steiner tree. The diameter of a k -Steiner forest is referred to as the maximum distance between two terminals of a branch. This paper studies the minimum diameter k-Steiner forest problem (MD k SFP) and establishes the relationship between MD k SFP and the absolute k-Steiner center problem (A k SCP). We first obtain a 2-factor dual approximation algorithm for A k SCP, and then achieve a 2-approximation algorithm for MD k SFP based on the 2-approximation to A k SCP. Furthermore, we develop an improved 2 ρ -approximation algorithm for MD k SFP, where ρ < 1 in general, by perturbing the sites of facilities and re-clustering the terminals. [ABSTRACT FROM AUTHOR]

For the underlying graph G of a network, k (⩾ 2) spanning trees of G are called completely independent spanning trees (CISTs for short) if they are mutually inner-node-disjoint. It is well-known that determining the existence of k CISTs in a graph is an NP-hard problem, even for k = 2. Also, there exists a k -connected graph which does not possess two CISTs for any k ⩾ 2. Accordingly, researches focused on the problem of constructing multiple CISTs in some particular famous networks. Pai and Chang (2016) proposed a unified approach to recursively construct two CISTs with diameter 2 n − 1 in several n -dimensional hypercube-variant networks for n ⩾ 4 , including locally twisted cubes L T Q n and crossed cubes C Q n. Later on, new constructions showed that the diameter of two CISTs can be reduced to 2 n − 2 if n = 4 and 2 n − 3 if n ⩾ 5 for L T Q n , and to 2 n − 2 if n = 4 , 5 and 2 n − 3 if n ⩾ 6 for C Q n. In this paper, we intend to construct more CISTs for those networks that are paid attention. We develop a novel tree searching algorithm, called two-stages tree-searching algorithm (abbreviated as TS 2), which can be used to construct three CISTs of a 6-regular graph if the scale of the graph is not too large and there exist three CISTs for certain. In particular, we demonstrate that three CISTs of L T Q 6 and C Q 6 can be acquired by using TS 2. Moreover, for high-dimensional L T Q n and C Q n with n ⩾ 7 , we show that their three CISTs can be constructed by recursion. Consequently, we obtain the following results: (i) For L T Q n , the diameters of three CISTs we constructed are 11, 12 and 12 when n = 6 , and all are 2 n − 1 when n ⩾ 7 ; (ii) For C Q n , all diameters of three CISTs we constructed are 12 when n = 6 , and 2 n + 1 when n ⩾ 7. [ABSTRACT FROM AUTHOR]