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On the Edge Connectivity of Semi-strong Product Graphs

Received: 9 September 2025     Accepted: 23 October 2025     Published: 11 December 2025
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Abstract

The concept of edge connectivity was first proposed by K. Menger, and in communication networks and logical networks, edge connectivity can be used to measure network reliability and fault tolerance. The graph product method can be used to construct complex networks, simulate biological molecule interactions etc. At present, research on the edge connectivity of product graphs mainly focuses on the connectivity of standard product graphs, such as Cartesian product graphs, strong product graphs. The unique properties exhibited by non standard product graphs (such as semi-strong product graphs.) in practical applications are worth further exploration. The concept of semi-strong product was proposed by Mordeson and Chang Shyh, that is, for two graphs and , their semi-strong product is a graph whose vertex set is , and the edge set is defined as follows: if and are two vertices in the semi-strong product , then there is an edge between them if and only if and and are adjacent in , or and are adjacent in and and are adjacent in . And applications of the semi-strong product in fuzzy graphs, symbolic graphs, and finance have shown its broad research prospects. In this article, we mainly study the edge connectivity of semi-strong product graphs, and obtain some exact values. Furthermore, we also give an necessary and sufficient condition for a semi-strong product to be maximally edge-connected.

Published in Applied and Computational Mathematics (Volume 14, Issue 6)
DOI 10.11648/j.acm.20251406.15
Page(s) 356-359
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This is an Open Access article, distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution and reproduction in any medium or format, provided the original work is properly cited.

Copyright

Copyright © The Author(s), 2025. Published by Science Publishing Group

Keywords

Edge Connectivity, Graph Product, Semi-strong Product

1. Introduction
The concept of edge connectivity was first proposed by K. Menger in 1930s, and a systematic research was obtained by means of Menger's theorem . Menger's theorem shows that the edge connectivity of a graph is equal to its minimum edge-cut set. This theory lays the foundation for the study of edge connectivity.
With the rapid development of information network, the reliability of network has become one of the key issues in research. Network reliability is usually measured by the connectivity of a graph, such as the edge connectivity is a key parameters to measure network reliability. For example, in communication networks and logical networks, edge connectivity can be used to measure network reliability and fault tolerance. Edge connectivity is defined as the minimum number of edge deletions required to make a graph disconnected, the concept is particularly important in the design of multiprocessor networks. In addition, edge connectivity is closely related to other properties of graphs, such as diameter, minimum degree, etc. These relationships provide theoretical support for the design and optimization of complex networks.
Product graph is an important construction method in graph theory, which combines the vertices and edges of two or more graphs to generate a new graph. The graph product method can be used to construct complex networks, simulate biological molecule interactions, optimize coding algorithms, and analyze relationships in data visualization. At present, research on the edge connectivity of product graphs mainly focuses on the edge connectivity of standard product graphs, such as Cartesian product graphs, strong product graphs, dictionary product graphs etc.
All graphs in this paper are finite and simple. Let G=(VG,EG) be a simple connected graph, where VG is the set of vertices of G and EG is the set of edges of G. For two graphs G1=(VG1,EG1) and G2=(VG2,EG2), their Cartesian product G1G2 is a graph whose vertex set is VG1G2=V(G1)×V(G2), and the edge set EG1G2 is defined as follows: if (x1, y1) and (x2, y2) are two vertices in the Cartesian product G1G2, then there is an edge between them if and only if x1=x2 and y1 and y2 are adjacent in G2, or x1 and x2 are adjacent in G1 and y1=y2; their strong product G1G2 is a graph with whose vertex set is VG1G2=V(G1)×V(G2), and the edge set EG1G2 is defined as follows: if (x1, y1) and (x2, y2) are two vertices in the strong product G1G2, then there is an edge between them if and only if x1=x2 and y1 and y2 are adjacent in G2, or x1 and x2 are adjacent in G1 and y1=y2, or x1 and x2 are adjacent in G1 and y1 and y2 are adjacent in G2; their lexicographic product G1G2 is a graph with whose vertex set is VG1G2=V(G1)×V(G2), and the edge set EG1G2 is defined as follows: if (x1, y1) and (x2, y2) are two vertices in the lexicographic product G1G2, then there is an edge between them if and only if x1=x2 and y1 and y2 are adjacent in G2, or x1 and x2 are adjacent in G1.
Let G=(VG,EG) be a simple connected graph. For any vertex v of a graph G, its vertex-neighbor set is denoted by NG(v), which is the set of all vertices adjacent to vertex v in the graph G. The degree of v in the graph G is denoted by dGv=d(v), which refers to the number of vertices related to v in the graph G. Therefore, according to the definition of the graph, we have dGv=NG(v), and the vertices in the graph G with degree 0 are called isolated vertices. The minimum degree of the graph G is denoted by δG=min{dGv:vV(G)}. If AG and A contains all edges uv in E(G) that satisfy {u,v}V(A), we say that A is a induced subgraph of G. Let F be a edge subset of E(G), if the induced subgraph of G-F is not connected, then F is a edge-cut set of G. The edge connectivity of the graph G, denoted by λ(G), is the cardinality of the minimum edge-cut set of graph G. For a connected graph G, it is said to be maximally edge-connected if and only if λG=δ(G). For two graphs G1 and G2, we use vi, ei, δi, λi and Vi to denote the number of vertices, the number of edges, the minimum degree, the edge connectivity and the vertex set of Gi, respectively, where i=1, 2.
For the edge connectivity of Cartesian product, Chiue and Shieh first gave a result in , that is, for two connected graphs G1 and G2, we have λG1G2λG1+λ(G2), and then there has been no good progress. Until Xu and Yang improved the result λG1G2λG1+λ(G2) in the previous study , a better result was obtained, that is, for two nontrivial graphs G1 and G2, We have λG1G2=min{λ1v2, λ2v1,δ1+δ2}, and in this article, the author use the edge-version of Menger's theorem to prove the result, then, Klavzar and Spacapan gave a proof in based on the set of minimum edge-cut . For the edge connectivity of strong product, Researchers proved that for two connected graph G1 and G2, we have λG1G2=min{λ1(v2+2e2), λ2(v1+2e1),δ1+δ2+δ1δ2} . For the edge connectivity of lexicographic product, Yang and Xu proved that for two graphs G1 and G2, we have λG1G2=min{2λ1v22,δ2+δ1v2} .
The graph product also includes some non-standard products, such as semi strong product etc. The concept of semi-strong product was proposed by Mordeson and Chang Shyh , and its applications in fuzzy graphs, symbolic graphs, and finance have shown its broad research prospects .
For two graphs G1=(VG1,EG1) and G2=(VG2,EG2), their semi-strong product G1G2 is a graph whose vertex set is VG1G2=V(G1)×V(G2), and the edge set E(G1G2) is defined as follows: if (x1, y1) and (x2, y2) are two vertices in the semi-strong product G1G2, then there is an edge between them if and only if x1=x2 and y1 and y2 are adjacent in G2, or x1 and x2 are adjacent in G1 and y1 and y2 are adjacent in G2.
In this paper, we study the edge connectivity of semi-strong product graphs. Given two graphs G1 and G2, we obtain that λG1G2=min{2λ1e2, δ2+δ1δ2}. In further, we also give an necessary and sufficient condition for a semi-strong product to be maximally edge-connected.
2. Preliminaries
For the convenience, we define the following two types of subgraphs. For x1V(G1), we define x1G2=G2x1={x1,y1|y1V(G2)}, it is clear that the subgraph of G1G2 induced by G2x1 is isomorphic to G2. Analogously, for y1V(G2), we define G1y1=G1y1={x1,y1|x1V(G1)}, but it is important to note that the subgraph of G1G2 induced by G1y1 is not isomorphic to G1.
Lemma 2.1 (). For any connected undirected graph G, we have λ(G)δ(G).
Lemma 2.2. Let G1 and G2 be two graphs, then δG1G2=δ2+δ1δ2.
Proof. According to the definition of semi-strong product graph, that is, the semi-strong product G1G2 of two graphs G1 and G2 is the graph with the vertex set V(G1)×V(G2), two vertices (x1, y1) and (x2, y2) being adjacent in G1G2 if and only if x1=x2 and y1y2 is an edge in G2, or x1x2 is an edge in G1 and y1y2 is an edge in G2, we can get the result of
δ(G1G2)δ2+δ1δ2.(1)
Assume that
δG1G2>δ2+δ1δ2,(2)
is true, that is
δG1G2δ2+δ1δ2+1.(3)
In this case, the product graph will rise to a new edge-connected relation: for two vertices (x1, y1) and (x2, y2), they are adjacent in G1G2 if and only if x1=x2 and y1y2 is an edge in G2, or x1x2 is an edge in G1 and y1y2 is an edge in G2, or x1x2 is an edge in G1 and y1=y2. At this point, three kinds of connected edges constitute a strong product graph, a contradiction. Therefore,
δG1G2=δ2+δ1δ2.(4)
3. Main Results
Theorem 3.1. Let G1 and G2 be two nontrivial graphs, and G1 is connected, then λG1G2=min{2λ1e2, δ2+δ1δ2}.
Proof. Let G=G1G2. It is clear that
λGδG= δ2+δ1δ2.(5)
Let S' be a minimum cut-set of G1 and there are two connected components C1' and C2' of G1-S'. Then we can see that in G there are exactly 2λ1e2 edges between C1'×V2 and C2'×V2. Hence λG2λ1e2. Then we have
λG1G2min{2λ1e2, δ2+δ1δ2}.(6)
Next we only need to prove that
λG1G2min{2λ1e2, δ2+δ1δ2}.(7)
Let S be a minimum edge-cut of G, C1 and C2 are two connected components of G-S. For xV1, let G2x denote the subgraph of G induced by xV2.
It is easy to see that G2x is isomorphic to G2. Then we define two sets, of which X={xV(G1)|xyV(C1)} for some yV(G2) and Y={xV(G1)|xyV(C2)} for some yV(G2).
It is clear that X and Y. If XY=, then it is easy to know that X,Y is a partition of VG1. Thus
SxyEG1X,YEGVG2x,VG2y
=EG1X,Y2e2=2λ1e2.(8)
So we assume that XY and let x0XY.
We need to know that for each neighbor x of x0, the vertex-set of the graph is VG2x0VG2x, and the edge-set between the vertex-sets VG2x0 and VG2x is E(VG2x0VG2x), denoted by G2[x0,x], has edge-connectivity δ2.
Let Sx0x=SE(VG2x0VG2x), then Sx0xδ2 for each neighbor x of x0; otherwise G2x0-S is connected through G2x, which contradicts to our hypothesis.
In the following, we claim that |Sx0x|+|Sx0|δ2+δ2=2δ2, of which Sx0=SE(VG2x0) and x is a neighbor of x0. Then let M=VG2x0V(C1), N=VG2x0V(C2), and we assume that MN.
Case 1: |M|=1.
If |M|=1, then |Sx0x|+|Sx0|δ2+δ2=2δ2 holds, since Sx0δ2.
Case 2: |M|2.
If |M|2, we can find |M|δ2 edge-disjoint (M,N)-paths in G2x0. Let M=s1,s2,,su and {t1,t2,,tu}N. Then for each i(1iu), there are δi edge-disjoint (si,ti) -paths in G2[x0,x]: (si,xz,ti) with zV(G2). So we can find |M|δ2 edge-disjoint (M,N)-paths.
For the sake of disconnected M from N, we must have |Sx0x||M|δ22δ2 and |Sx0x|+|Sx0|δ2+δ2=2δ2 also holds.
Let x1,x2,,xδ1 be δ1 neighbors of x0 in G1, then
SSx0x+Sx0+i=2δ1Sx0,xi
=δ2+δ2+δ1-1δ2=δ2+δ1δ2.(9)
This completes the proof.
Corollary 3.1. Let G1 and G2 be two connected graphs, then G1G2 is maximally edge connected if and only if 2λ1e2δ2+δ1δ2.
4. Conclusions
The main conclusion of this paper is as follows: for two nontrivial graphs G1 and G2, and G1 is connected, then we have λG1G2=min{2λ1e2, δ2+δ1δ2}. Furthermore, we obtain an necessary and sufficient condition for a semi-strong product to be maximally edge-connected.
At present, research on the edge connectivity of product graphs mainly focuses on the edge connectivity of standard product graphs, such as Cartesian product graphs, strong product graphs, lexicographic product graphs etc. The graph product also includes some non-standard products, such as semi-strong products, etc. The unique properties exhibited by non-standard product graphs (such as semi-strong product graphs, etc.) in practical applications are worth further exploration.
Author Contributions
Qiaoling Wang: Conceptualization, Resources, Software, Formal Analysis, Validation, Investigation, Visualization, Methodology, Writing – original draft
Haizhen Ren: Data curation, Supervision, Funding acquisition, Project administration, Writing – review & editing
Funding
This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 12161073).
Data Availability Statement
The data supporting the outcome of this research work has been reported in this manuscript.
Conflicts of Interest
The authors declare no conflicts of interest.
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    Wang, Q., Ren, H. (2025). On the Edge Connectivity of Semi-strong Product Graphs. Applied and Computational Mathematics, 14(6), 356-359. https://doi.org/10.11648/j.acm.20251406.15

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    Wang, Q.; Ren, H. On the Edge Connectivity of Semi-strong Product Graphs. Appl. Comput. Math. 2025, 14(6), 356-359. doi: 10.11648/j.acm.20251406.15

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    Wang Q, Ren H. On the Edge Connectivity of Semi-strong Product Graphs. Appl Comput Math. 2025;14(6):356-359. doi: 10.11648/j.acm.20251406.15

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  • @article{10.11648/j.acm.20251406.15,
      author = {Qiaoling Wang and Haizhen Ren},
      title = {On the Edge Connectivity of Semi-strong Product Graphs},
      journal = {Applied and Computational Mathematics},
      volume = {14},
      number = {6},
      pages = {356-359},
      doi = {10.11648/j.acm.20251406.15},
      url = {https://doi.org/10.11648/j.acm.20251406.15},
      eprint = {https://article.sciencepublishinggroup.com/pdf/10.11648.j.acm.20251406.15},
      abstract = {The concept of edge connectivity was first proposed by K. Menger, and in communication networks and logical networks, edge connectivity can be used to measure network reliability and fault tolerance. The graph product method can be used to construct complex networks, simulate biological molecule interactions etc. At present, research on the edge connectivity of product graphs mainly focuses on the connectivity of standard product graphs, such as Cartesian product graphs, strong product graphs. The unique properties exhibited by non standard product graphs (such as semi-strong product graphs.) in practical applications are worth further exploration. The concept of semi-strong product was proposed by Mordeson and Chang Shyh, that is, for two graphs  and , their semi-strong product  is a graph whose vertex set is , and the edge set  is defined as follows: if  and  are two vertices in the semi-strong product , then there is an edge between them if and only if  and  and  are adjacent in , or  and  are adjacent in  and  and  are adjacent in . And applications of the semi-strong product in fuzzy graphs, symbolic graphs, and finance have shown its broad research prospects. In this article, we mainly study the edge connectivity of semi-strong product graphs, and obtain some exact values. Furthermore, we also give an necessary and sufficient condition for a semi-strong product to be maximally edge-connected.},
     year = {2025}
    }
    

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    T2  - Applied and Computational Mathematics
    JF  - Applied and Computational Mathematics
    JO  - Applied and Computational Mathematics
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    UR  - https://doi.org/10.11648/j.acm.20251406.15
    AB  - The concept of edge connectivity was first proposed by K. Menger, and in communication networks and logical networks, edge connectivity can be used to measure network reliability and fault tolerance. The graph product method can be used to construct complex networks, simulate biological molecule interactions etc. At present, research on the edge connectivity of product graphs mainly focuses on the connectivity of standard product graphs, such as Cartesian product graphs, strong product graphs. The unique properties exhibited by non standard product graphs (such as semi-strong product graphs.) in practical applications are worth further exploration. The concept of semi-strong product was proposed by Mordeson and Chang Shyh, that is, for two graphs  and , their semi-strong product  is a graph whose vertex set is , and the edge set  is defined as follows: if  and  are two vertices in the semi-strong product , then there is an edge between them if and only if  and  and  are adjacent in , or  and  are adjacent in  and  and  are adjacent in . And applications of the semi-strong product in fuzzy graphs, symbolic graphs, and finance have shown its broad research prospects. In this article, we mainly study the edge connectivity of semi-strong product graphs, and obtain some exact values. Furthermore, we also give an necessary and sufficient condition for a semi-strong product to be maximally edge-connected.
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