19,402 research outputs found
Cyclic congruences of slim semimodular lattices and non-finite axiomatizability of some finite structures
summary:We give a new proof of the fact that finite bipartite graphs cannot be axiomatized by finitely many first-order sentences among finite graphs. (This fact is a consequence of a general theorem proved by L. Ham and M. Jackson, and the counterpart of this fact for all bipartite graphs in the class of all graphs is a well-known consequence of the compactness theorem.) Also, to exemplify that our method is applicable in various fields of mathematics, we prove that neither finite simple groups, nor the ordered sets of join-irreducible congruences of slim semimodular lattices can be described by finitely many axioms in the class of finite structures. Since a 2007 result of G. Grätzer and E. Knapp, slim semimodular lattices have constituted the most intensively studied part of lattice theory and they have already led to results even in group theory and geometry. In addition to the non-axiomatizability results mentioned above, we present a new property, called Decomposable Cyclic Elements Property, of the congruence lattices of slim semimodular lattices
Quantum contextual finite geometries from dessins d'enfants
We point out an explicit connection between graphs drawn on compact Riemann
surfaces defined over the field of algebraic numbers ---
so-called Grothendieck's {\it dessins d'enfants} --- and a wealth of
distinguished point-line configurations. These include simplices,
cross-polytopes, several notable projective configurations, a number of
multipartite graphs and some 'exotic' geometries. Among them, remarkably, we
find not only those underlying Mermin's magic square and magic pentagram, but
also those related to the geometry of two- and three-qubit Pauli groups. Of
particular interest is the occurrence of all the three types of slim
generalized quadrangles, namely GQ(2,1), GQ(2,2) and GQ(2,4), and a couple of
closely related graphs, namely the Schl\"{a}fli and Clebsch ones. These
findings seem to indicate that {\it dessins d'enfants} may provide us with a
new powerful tool for gaining deeper insight into the nature of
finite-dimensional Hilbert spaces and their associated groups, with a special
emphasis on contextuality.Comment: 18 page
An application of Hoffman graphs for spectral characterizations of graphs
In this paper, we present the first application of Hoffman graphs for
spectral characterizations of graphs. In particular, we show that the
-clique extension of the -grid is determined by its
spectrum when is large enough. This result will help to show that the
Grassmann graph is determined by its intersection numbers as a
distance regular graph, if is large enough
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