Grishin algebras are a generalisation of Boolean algebras that provide algebraic models for classical bilinear logic with two mutually cancelling negation connectives. We show how to build complete Grishin algebras as algebras of certain subsets (“propositions”) of cover systems that use an orthogonality relation to interpret the negations. The variety of Grishin algebras is shown to be closed ...

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A finite cover C of a group G is a finite collection of proper subgroups of G such that G is equal to the union of all of the members of C. Such a cover is called minimal if it has the smallest cardinality among all finite covers of G. The covering number of G, denoted by σ(G), is the number of subgroups in a minimal cover of G. In this paper the covering number of the Mathieu group M24 is show...

The Antarctic contribution to sea-level rise has long been uncertain. While regional variability in ice dynamics has been revealed, a picture of mass changes throughout the continental ice sheet is lacking. Here, we use satellite radar altimetry to measure the elevation change of 72% of the grounded ice sheet during the period 1992-2003. Depending on the density of the snow giving rise to the o...

We show that hitting set and set cover problems with half-strips oriented in two opposite directions are NPcomplete.

This paper investigates whether investors are compensated for the tax burden of equity securities. Effective tax rates on equity securities vary over time due to frequent tax reforms and cross-sectionally due to persistent differences in propensities to pay dividends. The paper finds an economically and statistically significant relationship between risk-adjusted stock returns and effective per...

We study the response of a two-dimensional hexagonal packing of rigid, frictionless spherical grains due to a vertically downward point force on a single grain at the top layer. We use a statistical approach, where each configuration of the contact forces is equally likely. We find that the response is double-peaked, independantly of the details of boundary conditions. The two peaks lie precise...

Let Γ denote the spaces of all double entire sequences. Let Λ denote the spaces of all double analytic sequences. This paper is devoted to a study of the general properties of Nörlund double entire sequence space η ( Γ ) , Γ and also study some of the properties of η ( Γ ) and η ( Λ ) .

We show that any graph that contains k edge-disjoint double rays for any k ∈ N contains also infinitely many edge-disjoint double rays. This was conjectured by Andreae in 1981.