Restricted Arc-Connectivity of Harary Digraphs

The h-restricted arc-connectivity of a digraph is an important parameter to measure fault-tolerance of interconnection networks.This paper determines that the h-restricted arc-connectivity of the Harary digraph D=G(n;1,2,.:,k)is equal to n/2 for 2≤h≤n/2,k=2 and n iseven,andλ_(h)(D)=g(k-1)for 2<h≤g and 3≤k≤n/2,where g is the girth of D.As consequences,the super restricted arc-connectedness of Harary digraph D is obtained immediately.In particular,for k=2 and n is even or 3≤k<n/2and n can be divided by k,it can be determined that distinct positive(respectively,negative)Ah-superatoms of D are vertex disjoint for 2≤h≤g.

A Classification of Spectrum-determined Circulant Digraphs

Some researchers have proved that Adam＇s conjecture is wrong. However, under special conditions,it is right. Let Zn be a cyclic group of order n and Cn（S） be the circulant digraph of Zn with respect to S？Zn／{0}. In the literature, some people have used a spectral method to solve the isomorphism for the circulants of prime-power order. In this paper, we also use the spectral method to characterize the circulants of order paqbωc（where p,q and ω are all distinct primes）, and to make Adam＇s conjecture right.

Conditional Edge Connectivity of the Locally Twisted Cubes

The k-component edgeconnectivity cλk(G)of anon-completegraph G is themini mum number of edges whose deletion results in a graph with at least k components In this paper,we extend some results by Guo et al.(Appl Math Comput 334:401-406,2018)by determining the component edge connectivity of the locally twisted cubes LTQn,i.e.,cλk+1(LTQn)=kn-exk/2for 1≤2[n/2],n≥7,where exk=∑si=0ti2ti+∑si=02·i·2ti,and k is a positive integer with decomposition k=∑si=02ti such that to="log2k"and ti="log2(k-∑i-1 r=02tr)"for i≥1.As a by-product,we characterize the corresponding optimal solutions.

On the Sizes of k-edge-maximal r-uniform Hypergraphs

Let H=(V,E)be a hypergraph,where V is a set of vertices and E is a set of non-empty subsets of V called edges.If all edges of H have the same cardinality r,then H is an r-uniform hypergraph;if E consists of all r-subsets of V,then H is a complete r-uniform hypergraph,denoted by K_(n)^(r),where n=|V|.A hypergraph H′=(V′,E′)is called a subhypergraph of H=(V,E)if V′⊆V and E′⊆E.The edge-connectivity of a hypergraph H is the cardinality of a minimum edge set F⊆E such that H−F is not connected,where H−F=(V,E\F).An r-uniform hypergraph H=(V,E)is k-edge-maximal if every subhypergraph of H has edge-connectivity at most k,but for any edge e∈E(K_(n)^(r))\E(H),H+e contains at least one subhypergraph with edge-connectivity at least k+1.Let k and r be integers with k≥2 and r≥2,and let t=t(k,r)be the largest integer such that(t−1 r−1)≤k.That is,t is the integer satisfying(t−1 r−1)≤k<(t r−1).We prove that if H is an r-uniform k-edge-maximal hypergraph such that n=|V(H)|≥t,then(i)|E(H)|≤(t r)+(n−t)k,and this bound is best possible;(ii)|E(H)|≥(n−1)k−((t−1)k−(t r))[n/t],and this bound is best possible.

Correction to“A Classification of Spectrum-determined Circulant Digraphs”

We find an error in the proof of Lemma 4 in[3].This makes that Theorem 2 and Corollary 3 in[3]are also wrong,and the next counterexample shows this.