The colours of BL Lac objects: a new approach to their classification

We selected a sample of 437 BL Lac objects, taken from the RomaBZCat catalogue, for which spectroscopic information and SDSS photometry is available. We propose a new classification of BL Lacs in which the sources' type is not defined only on the basis of the peak frequency of the synchrotron component in their Spectral Energy Distribution (types L and H), but also on the relevance of this component with respect to the brightness of the host galaxy (types N and G, for nuclear or galaxy dominated sources). We found that the SDSS colour index u-r=1.4 is a good separator between these two types. We used multiband colour-colour plots to study the properties of the BL Lac classes and found that in the X-ray to radio flux ratio vs u-r plot most of the N (blue) sources are located in a rather narrow strip, while the G-sources (red) are spread in a large area, and most of them are located in galaxy clusters or interacting systems, suggesting that their X-ray emission is not from a genuine BL Lac nucleus but it is related to their environment. Of the about 135 sources detected in the gamma-rays by Fermi-GST, nearly all belong to the N-type, indicating that only this type of sources should be considered as genuine BL Lac nuclei. The J-H, H-K plot of sources detected in the 2MASS catalogue is consistent with that of the "bona fide" BL Lac objects, independently of their N or G classification from the optical indices, indicating the existence in G-type sources of a K-band excess possibly due to a steep, low frequency peaked emission which deserves further investigations. We propose to use these colour plots as a further tool for searching candidate counterparts of newly discovered high-energy sources.Comment: 11 pages, 7 figures. Submitted 29/08/2011 to MNRAS, first referee report received 31/10/2011, accepted 21/02/201

On bounding the difference between the maximum degree and the chromatic number by a constant

We provide a finite forbidden induced subgraph characterization for the graph class $\varUpsilon_k$, for all $k \in \mathbb{N}_0$, which is defined as follows. A graph is in $\varUpsilon_k$ if for any induced subgraph, $\Delta \leq \chi -1 + k$ holds, where $\Delta$ is the maximum degree and $\chi$ is the chromatic number of the subgraph. We compare these results with those given in [O. Schaudt, V. Weil, On bounding the difference between the maximum degree and the clique number, Graphs and Combinatorics 31(5), 1689-1702 (2015). DOI: 10.1007/s00373-014-1468-3], where we studied the graph class $\varOmega_k$, for $k \in \mathbb{N}_0$, whose graphs are such that for any induced subgraph, $\Delta \leq \omega -1 + k$ holds, where $\omega$ denotes the clique number of a graph. In particular, we give a characterization in terms of $\varOmega_k$ and $\varUpsilon_k$ of those graphs where the neighborhood of every vertex is perfect.Comment: 10 pages, 4 figure

Wilhelm Brinkmann/Jörg Petersen (Hrsg.): Theorien und Modelle der Allgemeinen Pädagogik. Eine Orientierungshilfe für Studierende der Pädagogik und in der pädagogischen Praxis Tätige. Donauwörth: Auer 1998. [Rezension]

Rezension von: Wilhelm Brinkmann/Jörg Petersen (Hrsg.): Theorien und Modelle der Allgemeinen Pädagogik. Eine Orientierungshilfe für Studierende der Pädagogik und in der pädagogischen Praxis Tätige. Donauwörth: Auer 1998. 384 S

Status und Schutz der Nymphenfledermaus in Sachsen-Anhalt

Die Nymphenfledermaus Myotis alcathoe (von Helversen und Heller, 2001) wurde erstmals für Griechenland und Ungarn beschrieben (von Helversen et al.2001). 2005 gelang der erste Artnachweis für Deutschland im Südwesten von Baden- Württemberg (Brinkmann & Niermann 2007, Brinkmann et al. 2007). Am Südwesthang des Kyffhäusers, im thüringischen Gipskarst, wurde die Nymphenfledermaus 2006 durch Sauerbier et al. (2006) festgestellt. Über die Verbreitung der Art in Europa geben Niermann et al. (2007) und in Sachsen-Anhalt Ohlendorf & Funkel (2008) Auskunft. Die Art kommt von Spanien (Niermann et al. 2007) bis Rumänien (eigene Beobachtungen 2008) vor. Die nördlichsten Beobachtungen der Vorkommen in Europa gelingen gegenwärtig am Nordharzrand in Sachsen-Anhalt