Volume 59, Issue 2

Approaches for classifying resolvable balanced incomplete block designs (RBIBDs) are surveyed. The main approaches can roughly be divided into two types: those building up a design parallel class by parallel class and those proceeding point by point. With an algorithm of the latter type — and by refining ideas dating back to 1917 and the doctoral thesis by Pieter Mulder — it is shown that the list of seven known resolutions of 2-(28, 4, 1) designs is complete; these objects are also known as the resolutions of unitals on 28 points.

We give a complete solution of the following two problems: (1)For which (n, x) does there exist a pair of hexagon triple systems of order n having x inside triples in common?(2)For which (n, x) does there exist a pair of hexagon triple systems having x outside triples in common?

By an extended triangle, we mean a loop, a loop with an edge attached (known as a lollipop), or a copy of K 3 (known as a triangle). In this paper, we completely solve the problem of decomposing the graph (λK υ)+ into extended triangles for all possible number of loops.

Plants grown in large numbers for commercial purposes are usually set out on a regular grid, triangular, square or hexagonal, or possibly a rectangular grid. We need to understand their behaviour when they compete with each other for light, water and nutrients. In a greenhouse, there may be a lamp over each plant, half the plants having their lamps on and half off. The intensity of light falling on any particular plant is determined mainly by whether its own lamp is on, next by the number of its nearest neighbours whose lamps are on, and perhaps also by the number of its second-nearest neighbours whose lamps are on. Such arrays are also used to study competition among different types of plants and, with some extra restrictions, to design field layouts as well. The problem of constructing suitable arrays was introduced in [CORMACK, R. M.: Spatial aspects of competition beween individuals. In: Spatial and Temporal Analysis in Ecology (R. M. Cormack and J. K. Ord, eds.), International Co-operative Publishing House, Fairland, Maryland, USA, 1979] and discussed in [GATES, D. J.: Competition between two types of plants with specified neighbour configurations, Math. Biosci. 48 (1980), 195–209]. Here we consider only the square grid.

Let k be an integer. A 2-edge connected graph G is said to be goal-minimally k-elongated (k-GME) if for every edge uv ∈ E(G) the inequality d G−uv(x, y) > k holds if and only if {u, v} = {x, y}. In particular, if the integer k is equal to the diameter of graph G, we get the goal-minimally k-diametric (k-GMD) graphs. In this paper we construct some infinite families of GME graphs and explore k-GME and k-GMD properties of cages.

The eccentricity e(υ) of vertex υ is defined as a distance to a farthest vertex from υ. The radius of a graph G is defined as r(G) = $$ \mathop {\min }\limits_{u \in V(G)} $$ {e(u)}. We consider properties of unchanging the radius of a graph under two different situations: deleting an arbitrary edge and deleting an arbitrary vertex. This paper gives the upper bounds for the number of edges in such graphs.

In [PLOŠČICA, M.: Separation in distributive congruence lattices, Algebra Universalis 49 (2003), 1–12] we defined separable sets in algebraic lattices and showed a close connection between the types of non-separable sets in congruence lattices of algebras in a finitely generated congruence distributive variety and the structure of subdirectly irreducible algebras in . Now we generalize these results using the concept of separable mappings (defined on some trees) and apply them to some lattice varieties.

The notion of a relatively uniform convergence (ru-convergence) has been used first in vector lattices and then in Archimedean lattice ordered groups. Let G be an Archimedean lattice ordered group. In the present paper, a relative uniform completion (ru-completion) $$ G_{\omega _1 } $$ of G is dealt with. It is known that $$ G_{\omega _1 } $$ exists and it is uniquely determined up to isomorphisms over G. The ru-completion of a finite direct product and of a completely subdirect product are established. We examine also whether certain properties of G remain valid in $$ G_{\omega _1 } $$. Finally, we are interested in the existence of a greatest convex l-subgroup of G, which is complete with respect to ru-convergence.

We formulate a version of the conditional probability of an IF-event considering the Łukasiewicz operations with IF-sets. Also some properties of conditional probability are proved. The results are strengthenings of preceding ones published in [LENDELOVÁ, K.: Conditional IF-probability. In: Soft Methods for Integrated Uncertainty Modelling. Proceedings of the 2006 InternationalWorkshop on Soft Methods in Probability and Statistics (SMPS 2006), Bristol, UK, September 5–7, 2006. Adv. Soft Comput., Springer-Verlag, Berlin, 2006, pp. 275–283].