Line-graphs of cubic graphs are normal

A graph is called normal if its vertex set can be covered by cliques and also by stable sets, such that every such clique and stable set have non-empty intersection. This notion is due to Korner, who introduced the class of normal graphs as an extension of the class of perfect graphs. Normality has also relevance in information theory. Here we prove, that the line graphs of cubic graphs are normal.Comment: 16 pages, 10 figure

Selected Topics in Top Quark Physics

I discuss three different selected topics in top quark physics. The first topic concerns the next-to-next-to-leading order calculation of the hadroproduction of top quark pairs and the role of multiple polylogarithms in this calculation. I report on an ongoing next-to-next-to-leading order calculation of heavy quark pair production in hadron collisions where the loop--by--loop part of the calculation is about to be completed. Calculating the loop-by-loop part allows one to take a glimpse at the mathematical structure of the full NNLO calculation. The loop-by-loop contributions bring in a new class of functions introduced only eight years ago by the Russian mathematician Goncharov called multiple polylogarithms. The second topic concerns a next-to-leading order calculation of unpolarized top quark decays which are analyzed in cascade fashion $t \to b + W^+$ followed by $W^+ \to l^+ + \nu_l$. Finally, I present some next-to-leading order results on polarized top quark decays which are analyzed in the top quark rest system.Comment: 11 pages, 4 figures, Invited talk given at the 12th Regional Conference of Mathematical Physics, Islamabad, Pakistan, March 2006, to be published in the proceeding

Helicity Amplitudes and Angular Decay Distributions

I discuss how to obtain angular decay distributions for sequential cascade decays using helicity methods. The angular decay distributions follow from a reasonably simple master formula involving bilinear forms of helicity amplitudes and Wigner's $d$ functions. I discuss in some detail the issue of gauge invariance for off-shell gauge bosons. As a technical exercise I calculate the linear relation between the helicity amplitudes and the invariant amplitudes of semileptonic and rare baryon decays. I discuss two explicit examples of angular decay distributions for (i) the decay $t\to b+W^+(\to \ell^+\nu_\ell)$ (which leads to the notion of the helicity fractions of the $W^+$), and (ii) the sequential decay $\Lambda_b\to\Lambda(\to p\pi^-)+J/\psi(\to \ell^+\ell^-)$.Comment: 16 pages, 2 figures, Lecture given at the Helmholtz International Summer School "Physics of Heavy Quarks and Hadrons", Dubna, Russia, July 15--28, 2013 (to be published in the Proceedings

The Determinants of Corporate Debt Maturity Structure: Evidence from Czech Firms (in English)

This paper investigates the determinants of the maturity structure of Czech corporate debt. A theoretical section provides an overview of contemporary theories on corporate debt maturity structure. An empirical section describes an econometric model that shows that long-term debt increases with company size and leverage and asset maturity. The impact of growth options, collateralizable assets, corporate-tax rate, and company-level volatility proves statistically insignificant. Finally, the paper discusses the limitations of the results in terms of data, variables, and determinants.bank debt, bond debt, corporate debt maturity structure, long-term debt, short-term debt, transition economy

The determinants of corporate debt maturity structure: evidence from Czech firms

This paper investigates the determinants of the corporate debt maturity structure of Czech firms. The theoretical section provides an overview of contemporary theories on corporate debt maturity structure. The regression section describes an econometric model showing that the long-term debt increases with Firm size, Leverage and Asset maturity. The impact of Growth options, Collateralizable assets, Firm tax rate, and Firm level volatility has been found out as statistically insignificant. The portfolio analyses section of this paper shows the bank-based system pattern of financing of Czech firms, increasing importance of intra-group financing and increasing presence of Maturity matching principle. Finally, the paper discusses the limitations of the results in the field of data, variables, and determinants.corporate debt maturity structure; bank debt; bond debt; short-term debt; long-term debt; transition economy

Effects of growth conditions on postharvest Botrytis infection in Gerbera - a nursery comparison

Botrytis cinerea is one of the main postharvest problems in gerbera cut flowers. There are clear differences among growers in the percentage of flowers showing symptoms of Botrytis infection after harvest. Because the factors causing these differences are uncertain, cultivation parameters of twelve different growers were followed and a nursery comparison was carried out. Gerbera flowers ‘Dino’ were sampled twelve times; six times in autumn 2006 and six times in spring of 2007. At the nurseries climate (air temperature, relative humidity, light intensity and CO2 concentration) was logged, and plant density, plant age and other characteristics were monitored. Before each harvest Botrytis spores were trapped in the greenhouses and counted. After sampling, flowers were treated according to a standard transport simulation. Thereafter Botrytis infection was monitored, and statistical analyses were performed on the effect of the pre-harvest conditions on Botrytis infection in the postharvest stage. Clear influence of the spore level in the greenhouse was observed. When a sufficient number of spores were present in the greenhouse environment, the humidity level and the intensity and duration of irradiation had the strongest influence on infection. In general, factors those lead to a dry microclimate such as the use of ventilators, the use of supplemental lighting and a low plant density were related with a lower number of lesions on gerbera petals in the postharvest phase