Building blocks for the variety of absolute retracts

AbstractGiven a graph H with a labelled subgraph G, a retraction of H to G is a homomorphism r:H→G such that r(x)=x for all vertices x in G. We call G a retract of H. While deciding the existence of a retraction to a fixed graph G is NP-complete in general, necessary and sufficient conditions have been provided for certain classes of graphs in terms of holes, see for example Hell and Rival.For any integer k⩾2 we describe a collection of graphs that generate the variety ARk of graphs G with the property that G is a retract of H whenever G is a subgraph of H and no hole in G of size at most k is filled by a vertex of H. We also prove that ARk⊂NUFk+1, where NUFk+1 is the variety of graphs that admit a near unanimity function of arity k+1

Considerations in the design of large space structures

Several analytical studies of topics relevant to the design of large space structures are presented. Topics covered are: the types and quantitative evaluation of the disturbances to which large Earth-oriented microwave reflectors would be subjected and the resulting attitude errors of such spacecraft; the influence of errors in the structural geometry of the performance of radiofrequency antennas; the effect of creasing on the flatness of tensioned reflector membrane surface; and an analysis of the statistics of damage to truss-type structures due to meteoroids

Homes as machines: Exploring expert and public imaginaries of low carbon housing futures in the United Kingdom

Low carbon housing policies embody visions of the future that shape and constrain current choices between different technological pathways. These socio-technical imaginaries include expectations around new ways of living and interacting with technology, with implications for everyday lives. This paper investigates existing expert visions of low carbon housing, and explores these futures with members of the public; utilising empirical data from policy documents, expert interviews and public focus groups. Two competing expert visions of low carbon housing were identified: Passivhaus and Smart Homes. Whilst portrayed as divergent futures, both visions aimed to ‘design out’ the role of occupants, achieving emissions reductions through changes to the built environment and maintaining current lifestyles; a position that was reinforced by an imagined public that was unable or unwilling to accept the need for lifestyle change. This construction of the public did not consider the complex personal and cultural dimensions that influenced public acceptability of future housing: specifically surrounding themes of comfort, control and security that arose within the focus groups. The tensions arising between expert and public imaginaries highlight the difficulties that may surround any transition towards a low carbon future and demonstrate the need to work with, rather than around, the public

Two new pulsating hot subdwarf stars from the Edinburgh-Cape survey

We report the discovery of very rapid pulsations in two hot subdwarf stars from the Edinburgh- Cape blue object survey. The short periods, small amplitudes and multiperiodicity establish these stars as members of the class of rapidly-pulsating sdB stars. The spectrograms of both stars, however, show relatively strong He II 4686 and they are therefore more properly classified as sdOB. The light curve of EC 01541?1409 is dominated by two strong (?1 per cent) variations with frequencies near 7114 and 7870 ?Hz (periods near 140.6 and 127.1 s), though at least five frequencies are present with amplitudes above about 0.002 mag. The light curve of EC 22221?3152 appears to be generated by at least 10 frequencies in the range 5670-11850 ?Hz (about 175-85 s) with amplitudes between about 0.01 and 0.001 mag, including the first overtone of the strongest variation. Somewhat surprisingly, this number of frequencies is detectable in observing runs as short as 3 h, probably due to the fact that the detected frequencies are well-separated.IS

The ESO Key-Programme ``A Homogeneous Bright QSO Survey'' - I The Methods and the ``Deep'' Fields

This is the first paper in a series aimed at defining a statistically significant sample of QSOs in the range $15 < B < 18.75$ and $0.3 < z < 2.2$. The selection is carried out using direct plates obtained at the ESO and UK Schmidt Telescopes, scanned with the COSMOS facility and searched for objects with an ultraviolet excess. Follow-up spectroscopy, carried out at ESO La Silla, is used to classify each candidate. In this initial paper, we describe the scientific objectives of the survey; the selection and observing techniques used. We present the first sample of 285 QSOs ($M_B < -23$) in a 153 deg$^2$ area, covered by the six ``deep'' fields, intended to obtain significant statistics down $B \simeq 18.75$ with unprecedented photometric accuracy. From this database, QSO counts are determined in the magnitude range $17 < B < 18.75$.Comment: 21 pages uuencoded compressed postscript, to appear in Astronomy and Astrophysics Supplements, 199

Complexation of Cm(III) with the recombinant N-lobe of human serum transferrin studied by time-resolved laser fluorescence spectroscopy (TRLFS)

The complexation of Cm(III) with the recombinant N-lobe of human serum transferrin (hTf/2N) is investigated in the pH range from 4.0 to 11.0 using TRLFS. At pH [greater-than-or-equal] 7.4 a Cm(III) hTf/2N species is formed with Cm(III) bound at the Fe(III) binding site. The results are compared with Cm(III) transferrin interaction at the C-lobe and indicate the similarity of the coordination environment of the C- and N-terminal binding sites with four amino acid residues of the protein, two H2O molecules and three additional ligands (e.g. synergistic anions such as carbonate) in the first coordination sphere. Measurements at c(carbonate)tot = 0.23 mM (ambient carbonate concentration) and c(carbonate)tot = 25 mM (physiological carbonate concentration) show that an increase of the total carbonate concentration suppresses the formation of the Cm(III) hTf/2N species significantly. Additionally, the three Cm(III) carbonate species Cm(CO3)+, Cm(CO3)2- and Cm(CO3)33- are formed successively with increasing pH. In general, carbonate complexation is a competing reaction for both Cm(III) complexation with transferrin and hTf/2N but the effect is significantly higher for the half molecule. At c(carbonate)tot = 0.23 mM the complexation of Cm(III) with transferrin and hTf/2N starts at pH [greater-than-or-equal] 7.4. At physiological carbonate concentration the Cm(III) transferrin species II forms at pH [greater-than-or-equal] 7.0 whereas the Cm(III) hTf/2N species is not formed until pH > 10.0. Hence, our results reveal significant differences in the complexation behavior of the C-terminal site of transferrin and the recombinant N-lobe (hTf/2N) towards trivalent actinides

The Firefighter Problem: A Structural Analysis

We consider the complexity of the firefighter problem where b>=1 firefighters are available at each time step. This problem is proved NP-complete even on trees of degree at most three and budget one (Finbow et al.,2007) and on trees of bounded degree b+3 for any fixed budget b>=2 (Bazgan et al.,2012). In this paper, we provide further insight into the complexity landscape of the problem by showing that the pathwidth and the maximum degree of the input graph govern its complexity. More precisely, we first prove that the problem is NP-complete even on trees of pathwidth at most three for any fixed budget b>=1. We then show that the problem turns out to be fixed parameter-tractable with respect to the combined parameter "pathwidth" and "maximum degree" of the input graph