A digraph D is a 3-anti-circulant digraph, if for any four distinct vertices x1, x2, x3, x4 ∈ V (D), x1 → x2 ← x3 → x4 implies x4 → x1. In this paper, we characterize the structure of 3-anti-circulant digraphs containing a cycle factor and show that the structure is very close to semicomplete and semicomplete bipartite digraphs. Laborde et al. conjectured that every digraph has an independent s...

It is known that there exists a Minimum Distance Diagram (MDD) for circulant digraphs of degree two (or double-loop computer networks) which is an L-shape. Its description provides the diameter and the average distance of the graph on constant time. In this paper we clarify, justify and extend these diagrams to circulant digraphs of arbitrary degree by presenting monomial ideals as a natural to...

It has long been known that the number of spanning trees in circulant graphs with fixed jumps and n nodes satisfies a recurrence relation in n. The proof of this fact was algebraic (relating the products of eigenvalues of the graphs’ adjacency matrices) and not combinatorial. In this paper we derive a straightforward combinatorial proof of this fact. Instead of trying to decompose a large circu...

We consider the degree-diameter problem for undirected and directed circulant graphs. To date, attempts to generate families of large circulant graphs of arbitrary degree for a given diameter have concentrated mainly on the diameter 2 case. We present a direct product construction yielding improved bounds for small diameters and introduce a new general technique for “stitching” together circula...

The numerical solution of large and sparse nonsymmetric linear systems of algebraic equations is usually the most time consuming part of time-step integrators for differential equations based on implicit formulas. Preconditioned Krylov subspace methods using Strang block circulant preconditioners have been employed to solve such linear systems. However, it has been observed that these block cir...

The Circulant Travelling Salesman Problem (CTSP) is the problem of nding a minimum weight Hamiltonian cycle in a weighted graph with circulant distance matrix. The computational complexity of this problem is not known. In fact, even the complexity of deciding Hamiltonicity of the underlying graph is unkown. This paper presents necessary and suucient conditions for the existence of a Hamilto-nia...

Toeplitz systems can be solved efficiently by using iterative methods such as the conjugate gradient algorithm. If a suitable preconditioner is used, the overall cost of the method is O(n logn) arithmetic operations. Circulant matrices are frequently employed for the preconditioning of Toeplitz systems. They can be chosen as preconditioners themselves, or they can be used for the computation of...

We associate to any given circulant complex matrix C another one E(C) such that E(E(C)) = C∗ the transpose conjugate of C. All circulant Hadamard matrices of order 4 satisfy a condition C4 on their eigenvalues, namely, the absolute value of the sum of all eigenvalues is bounded above by 4. We prove by a “descent” that uses our operator E that the only circulant Hadamard matrices of order n > 4,...

For a set D ⊂ Zn, the distance graph Pn(D) has Zn as its vertex set and the edges are between vertices i and j with |i− j| ∈ D. The circulant graph Cn(D) is defined analogously by considering operations modulo n. The minimum feedback vertex set problem consists in finding the smallest number of vertices to be removed in order to cut all cycles in the graph. This paper studies the minimum feedba...

We investigate conditions for isomorphism between circulant graphs and analyze their automorphism group, with special emphasis on loop networks. We give a survey of pertinent published results and answer partially a long-standing conjecture [5] that all degree 4 circulants which are isomorphic are Ad am isomorphic. This paper also surveys solutions to several problems introduced in [9]. In part...