Highly Symmetric Subgraphs of Hypercubes

Two questions are considered, namely (i) How many colors are needed for a coloring of the n-cube without monochromatic quadrangles or hexagons? We show that four colors suffice and thereby settle a problem of Erdös. (ii) Which vertex-transitive induced subgraphs does a hypercube have? An interesting graph has come up in this context: If we delete a Hamming code from the 7-cube, the resulting graph is 6-regular, vertex-transitive and its edges can be two-colored such that the two monochromatic subgraphs are isomorphic, cubic, edge-transitive, nonvertex-transitive graphs of girth 10.     

Hausdorff metric on faces of the <Emphasis Type="Italic">n</Emphasis>-cube

The Hausdorff metric on all faces of the unit n-cube (I ⁿ ) is considered. The notion of a cubant is used; it was introduced as an n-digit quaternary code of a k-dimensional face containing the Cartesian product of k frame unit segments and the face translation code within I ⁿ . The cubants form a semigroup with a unit (monoid) with respect to the given operation of multiplication. A calculation of Hausdorff metric based on the generalization of the Hamming metric for binary codes is considered. The supercomputing issues are discussed.     

Gray Codes,Loopless Algorithm and Partitions

The generation of efficient Gray codes and combinatorial algorithms that list all the members of a combinatorial object has received a lot of attention in the last few years. Knuth gave a code for the set of all partitions of [n] = {1,2,...,n}. Ruskey presented a modified version of Knuth’s algorithm with distance 2. Ehrlich introduced a looplees algorithm for the set of the partitions of [n]; Ruskey and Savage generalized Ehrlich’s results and introduced two Gray codes for the set of partitions of [n]. In this paper, we give another combinatorial Gray code for the set of the partitions of [n] which differs from the aforementioned Gray codes. Also, we construct a different loopless algorithm for generating the set of all partitions of [n] which gives a constant time between successive partitions in the construction process.      

A characterization of truth-functions in the nilpotent minimum logic

By introducing a new family of partitions into the n-cube [0,1]ⁿ, the problem of characterizing truth tables of formulas in the nilpotent minimum logic is solved and their normal forms are presented. So far, only this kind of fuzzy truth functions have normal forms among all fuzzy propositional calculi which are based on left-continuous but discontinuous t-norm.     

Balancing the n-Cube: A Census of Colorings

Weights of 1 or 0 are assigned to the vertices of the n-cube in n-dimensional Euclidean space. Such an n-cube is called balanced if its center of mass coincides precisely with its geometric center. The seldom-used n-variable form of Pólya's enumeration theorem is applied to express the number N _{n, 2k} of balanced configurations with 2k vertices of weight 1 in terms of certain partitions of 2k. A system of linear equations of Vandermonde type is obtained, from which recurrence relations are derived which are computationally efficient for fixed k. It is shown how the numbers N _{n, 2k} depend on the numbers A _{n, 2k} of specially restricted configurations. A table of values of N _{n, 2k} and A _{n, 2k} is provided for n = 3, 4, 5, and 6. The case in which arbitrary, nonnegative, integral weights are allowed is also treated. Finally, alternative derivations of the main results are developed from the perspective of superposition.     

Enumerating Perfect Matchings in n-Cubes

The perfect matchings in the n-cube have earlier been enumerated for n?≤?6. A dynamic programming approach is here used to obtain the total number of perfect matchings in the 7-cube, which is 391 689 748 492 473 664 721 077 609 089. The number of equivalence classes of perfect matchings is further shown to be 336 in the 5-cube, 356 788 059 in the 6-cube and 607 158 046 495 120 886 820 621 in the 7-cube. The techniques used can be generalized to arbitrary bipartite and general graphs.     

Dominating sets in n-cubes

A perfectdominatingset S of a graph Γ is a set of vertices of Γ such that every vertex of Γ is either in S or is adjacent to exactly one vertex of S. We show that a perfect dominating set of the n-cube Q_n induces a subgraph of Q_n whose components are isomorphic to hypercubes. We conjecture that each of these hypercubes has the same dimension. We then prove that if Q_r is a component of the subgraph induced by S, then n ? r ? 1 or 3 (mod 6). A number of examples are given and connections with Steiner Systems and codes are noted.     

On the classification of the extremal self-dual codes over small fields with 2-transitive automorphism groups

There are seven binary extremal self-dual doubly-even codes which are known to have a 2-transitive automorphism group. Using representation theoretical methods we show that there are no other such codes, except possibly for length n = 1024. We also classify all extremal ternary self-dual and quaternary Hermitian self-dual codes.     

On orientation-preserving automorphisms of Hamiltonian cycles in the <Emphasis Type="Italic">N</Emphasis>-dimensional Boolean cube

We obtain an upper bound for the order of the group of orientation-preserving automorphisms of a Hamiltonian cycle in the Boolean n-cube. We prove that the existence of a Hamiltonian cycle with the order of the group of orientation-preserving automorphisms attaining this upper bound is equivalent to the existence of a Hamiltonian cycle with an additional condition on the graph of orbits of a fixed automorphism of the n-cube.     

Semisymmetric graphs from polytopes

Every finite, self-dual, regular (or chiral) 4-polytope of type {3,q,3} has a trivalent 3-transitive (or 2-transitive) medial layer graph. Here, by dropping self-duality, we obtain a construction for semisymmetric trivalent graphs (which are edge- but not vertex-transitive). In particular, the Gray graph arises as the medial layer graph of a certain universal locally toroidal regular 4-polytope.     

On Application of the Partition Distance Concept to a Comparative Analysis of Psychological or Sociological Tests

Abstract

We discuss two distance concepts between q-ary n-sequences, 2 ≤ q < n, called partition distances. This distances are metrics in the space of all partitions of a finite n-set. For the metrics, we study codes called q-partition codes and present a construction of these codes based on the first order Reed–Muller codes. A random coding bound is obtained. We also work out an application of q-partition codes to the statistical analysis of psychological or medical tests using questionnaires.     

Vertex-transitive graphs that are not Cayley graphs. II

The Petersen graph on 10 vertices is the smallest example of a vertex-transitive graph that is not a Cayley graph. In 1983, D. Marus˘ic˘ asked, “For what values of n does there exist such a graph on n vertices?” We give several new constructions of families of vertex-transitive graphs that are not Cayley graphs and complete the proof that, if n is divisible by p² for some prime p, then there is a vertex-transitive graph on n vertices that is not a Cayley graph unless n is p², p³, or 12. © 1996 John Wiley & Sons, Inc.     

1/2-Transitive Graphs of Order 3p

A graph X is called vertex-transitive, edge-transitive, or arc-transitive, if the automorphism group of X acts transitively on the set of vertices, edges, or arcs of X, respectively. X is said to be 1/2-transitive, if it is vertex-transitive, edge-transitive, but not arc-transitive.In this paper we determine all 1/2-transitive graphs with 3p vertices, where p is an odd prime. (See Theorem 3.4.)     

Constructions of partitions of FqN into perfect q-ary codes

Given N = (q ^m − 1)/(q − 1), where q is a power of a prime, q > 2, we present two constructions of different partitions of the set F _q ^N of all q-ary length N vectors into perfect q-ary codes of length N. The lower bounds on the number of these partitions are presented.     

Perfect Matchings of Cellular Graphs

We introduce a family of graphs, called cellular, and consider the problem of enumerating their perfect matchings. We prove that the number of perfect matchings of a cellular graph equals a power of 2 times the number of perfect matchings of a certain subgraph, called the core of the graph. This yields, as a special case, a new proof of the fact that the Aztec diamond graph of order n introduced by Elkies, Kuperberg, Larsen and Propp has exactly 2 ^n(n+1)/2 perfect matchings. As further applications, we prove a recurrence for the number of perfect matchings of certain cellular graphs indexed by partitions, and we enumerate the perfect matchings of two other families of graphs called Aztec rectangles and Aztec triangles.     

On Perfect 2‐Colorings of Johnson Graphs J(v, 3)

We study the perfect 2‐colorings (also known as the equitable partitions into two parts or the completely regular codes with covering radius 1) of the Johnson graphs . In particular, we classify all the realizable quotient matrices of perfect 2‐colorings for odd v. © 2012 Wiley Periodicals, Inc. J. Combin. Designs 21: 232–252, 2013     

Combining Perfect Systems of Difference Sets

We use sharply 2-transitive permutation groups to constructan additive sequence of permutations from a system of differencesets, each component of which has size one less than a primepower. This allows us to combine perfect systems of differencesets to form other perfect systems. In particular, if thereexists a perfect (m, n + 1, 1)-system and a perfect (q, n +1, 1)-system then there exists a perfect (mqn(n + 1) + m + q,n + 1, 1)-system.     

On the decomposition of n-cubes into isomorphic trees

We prove that if T is any tree having n edges (n ≥ 1), then the n-cube Q_n can be decomposed into 2^n-1 edge-disjoint induced subgraphs, each of which is isomorphic to T. We use this statement to obtain two results concerning decompositions of Q_n into subgraphs isomorphic to members of a specified family of trees.     

Combinatorial S >n-Modules as Codes

Certain -modules related to the kernels ofincidence maps between types in the poset defined by the natural productorder on the set of n-tuples with entries from {1, ,m} are studied as linear codes (whencoefficients are extended to an arbitrary field K). Theirdimensions and minimal weights are computed. The Specht modules areextremal among these submodules. The minimum weight codewords of theSpecht module are shown to be scalar multiples of polytabloids. Ageneralization of t-design arising from the natural permutationS _n-modules labelled by partitions with mparts is introduced. A connection with Reed-Muller codes is noted and acharacteristic free formulation is presented.     

Odd‐sized partitions of Russell‐sets

In the setting of ZF, i.e., Zermelo–Fraenkel set theory without the Axiom of Choice (AC), we study partitions of Russell‐sets into sets each with exactly n elements (called n ‐ary partitions), for some integer n. We show that if n is odd, then a Russell‐set X has an n ‐ary partition if and only if |X | is divisible by n. Furthermore, we establish that it is relative consistent with ZF that there exists a Russell‐set X such that |X | is not divisible by any finite cardinal n > 1 (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)