PERULANGAN PADA DIGRAF HAMPIR MOORE

Digraf Moore adalah graf berarah (directed graph) atau digraf yang memiliki derajat d, diameter k, dan jumlah titik sebanyak n = 1 + d + d 2 + ... + d k atau disebut sebagai jumlah Moore. Telah diketahui di (Plesnik & Znam, 1974), dan (Bidges & Toueg, 1980) bahwa digraf Moore hanya ada pada kasus-kasus trivial yaitu untuk d = 1 (digraf lingkaran Ck+1) dan untuk k = 1 (digraf lengkap Kd+1). Penelitian baru-baru ini diarahkan pada menentukan keberadaan digraf seperti di atas dengan jumlah titik kurang satu dari jumlah Moore atau disebut digraf hampir Moore dan ditulis sebagai (d,k)-digraf. Digraf yang memiliki jumlah titik seperti di atas mengakibatkan munculnya konsep perulangan dan perulangan-diri. Penelitian Miller & Fris (1992) mendapatkan bahwa (d,2)-digraf selalu ada. Pertanyaan yang sangat penting untuk dijawab adalah berapa banyak (d,2)-digraf yang memiliki struktur berbeda untuk d tertentu? Penulis dan peneliti yang lain mengunakan konsep perulangan dan perulangan-diri seperti di Baskoro, etal (1995), Simanjuntak & Baskoro, (1999), Iswadi & Baskoro, (1999) dan Baskoro, etal (1998) untuk menjawab sebagian pertanyaan di atas. Penelitian kali ini akan mengali lebih dalam sifatsifat titik perulangan dan perulangan-diri dengan mengembangkan hasil yang telah didapat di Iswadi & Baskoro, (1999)

On The Existence of Non-Diregular Digraphs of Order Two Less than the Moore Bound

A communication network can be modelled as a graph or a directed graph, where each processing element is represented by a vertex and the connection between two processing elements is represented by an edge (or, in case of directed connections, by an arc). When designing a communication network, there are several criteria to be considered. For example, we can require an overall balance of the system. Given that all the processing elements have the same status, the flow of information and exchange of data between processing elements will be on average faster if there is a similar number of interconnections coming in and going out of each processing element, that is, if there is a balance (or regularity) in the network. This means that the in-degree and out-degree of each vertex in a directed graph (digraph) must be regular. In this paper, we present the existence of digraphs which are not diregular (regular out-degree, but not regular in-degree) with the number of vertices two less than the unobtainable upper bound for most values of out-degree and diameter, the so-called Moore bound

Nonexistence of almost Moore digraphs of diameter four

Regular digraphs of degree d > 1, diameter k > 1 and order N(d; k) = d+ +dk will be called almost Moore (d; k)-digraphs. So far, the problem of their existence has only been solved when d = 2; 3 or k = 2; 3. In this paper we prove that almost Moore digraphs of diameter 4 do not exist for any degree dPostprint (published version

Ideal bases in constructions defined by directed graphs

The present article continues the investigation of visible ideal bases in constructions defined using directed graphs. Our main theorem establishes that, for every balanced digraph D and each idempotent semiring R with 1, the incidence semiring ID(R) of the digraph D has a convenient visible ideal basis BD(R). It also shows that the elements of BD(R) can always be used to generate two-sided ideals with the largest possible weight among the weights of all two-sided ideals in the incidence semiring

Ideal Basis in Constructions Defined by Directed Graphs

The present article continues the investigation of visible ideal bases in constructions defined using directed graphs. This notion is motivated by its applications for the design of classication systems. Our main theorem establishes that, for every balanced digraph and each idempotent semiring with identity element, the incidence semiring of the digraph has a convenient visible ideal basis. It also shows that the elements of the basis can always be used to generate ideals with the largest possible weight among the weights of all ideals in the incidence semiring

Multipartite Moore digraphs

We derive some Moore-like bounds for multipartite digraphs, which extend those of bipartite digraphs, under the assumption that every vertex of a given partite set is adjacent to the same number $\delta$ of vertices in each of the other independent sets. We determine when a Moore multipartite digraph is weakly distance-regular. Within this framework, some necessary conditions for the existence of a Moore $r$-partite digraph with interpartite outdegree $\delta>1$ and diameter $k=2m$ are obtained. In the case $\delta=1$, which corresponds to almost Moore digraphs, a necessary condition in terms of the permutation cycle structure is derived. Additionally, we present some constructions of dense multipartite digraphs of diameter two that are vertex-transitive

Structural properties and labeling of graphs

The complexity in building massive scale parallel processing systems has re- sulted in a growing interest in the study of interconnection networks design. Network design affects the performance, cost, scalability, and availability of parallel computers. Therefore, discovering a good structure of the network is one of the basic issues. From modeling point of view, the structure of networks can be naturally stud- ied in terms of graph theory. Several common desirable features of networks, such as large number of processing elements, good throughput, short data com- munication delay, modularity, good fault tolerance and diameter vulnerability correspond to properties of the underlying graphs of networks, including large number of vertices, small diameter, high connectivity and overall balance (or regularity) of the graph or digraph. The first part of this thesis deals with the issue of interconnection networks ad- dressing system. From graph theory point of view, this issue is mainly related to a graph labeling. We investigate a special family of graph labeling, namely antimagic labeling of a class of disconnected graphs. We present new results in super (a; d)-edge antimagic total labeling for disjoint union of multiple copies of special families of graphs. The second part of this thesis deals with the issue of regularity of digraphs with the number of vertices close to the upper bound, called the Moore bound, which is unobtainable for most values of out-degree and diameter. Regularity of the underlying graph of a network is often considered to be essential since the flow of messages and exchange of data between processing elements will be on average faster if there is a similar number of interconnections coming in and going out of each processing element. This means that the in-degree and out-degree of each processing element must be the same or almost the same. Our new results show that digraphs of order two less than Moore bound are either diregular or almost diregular.Doctor of Philosoph

On the structure of digraphs with order close to the Moore bound

The Moore bound for a diregular digraph of degree d and diameter k is M d;k = 1 + d + : : : + d k . It is known that digraphs of order M d;k do not exist for d ? 1 and k ? 1 ([24] or [6]). In this paper we study digraphs of degree d, diameter k and order M d;k \Gamma 1, denoted by (d; k)-digraphs. Miller and Fris showed that (2; k)- digraphs do not exist for k 3 [22]. Subsequently, we gave a necessary condition of the existence of (3; k)-digraphs, namely, (3; k)-digraphs do not exist if k is odd or if k + 1 does not divide 9 2 (3 k \Gamma 1) [3]. The (d; 2)-digraphs were considered in [4]. In this paper, we present further necessary conditions for the existence of (d; k)-digraphs. In particular, for d; k 3, we show that a (d; k)-digraph contains either no cycle of length k or exactly one cycle of length k. 1 Introduction By a digraph we mean a structure G = (V; A) where V (G) is a nonempty set of elements called vertices; and A(G) is a set of ordered pairs (u; v) of disti..