2,579 research outputs found
On the bend number of circular-arc graphs as edge intersection graphs of paths on a grid
Golumbic, Lipshteyn and Stern \cite{Golumbic-epg} proved that every graph can
be represented as the edge intersection graph of paths on a grid (EPG graph),
i.e., one can associate with each vertex of the graph a nontrivial path on a
rectangular grid such that two vertices are adjacent if and only if the
corresponding paths share at least one edge of the grid. For a nonnegative
integer , -EPG graphs are defined as EPG graphs admitting a model in
which each path has at most bends. Circular-arc graphs are intersection
graphs of open arcs of a circle. It is easy to see that every circular-arc
graph is a -EPG graph, by embedding the circle into a rectangle of the
grid. In this paper, we prove that every circular-arc graph is -EPG, and
that there exist circular-arc graphs which are not -EPG. If we restrict
ourselves to rectangular representations (i.e., the union of the paths used in
the model is contained in a rectangle of the grid), we obtain EPR (edge
intersection of path in a rectangle) representations. We may define -EPR
graphs, , the same way as -EPG graphs. Circular-arc graphs are
clearly -EPR graphs and we will show that there exist circular-arc graphs
that are not -EPR graphs. We also show that normal circular-arc graphs are
-EPR graphs and that there exist normal circular-arc graphs that are not
-EPR graphs. Finally, we characterize -EPR graphs by a family of
minimal forbidden induced subgraphs, and show that they form a subclass of
normal Helly circular-arc graphs
Fermionization of two distinguishable fermions
In this work we study a system of two distinguishable fermions in a 1D
harmonic potential. This system has the exceptional property that there is an
analytic solution for arbitrary values of the interparticle interaction. We
tune the interaction strength via a magnetic offset field and compare the
measured properties of the system to the theoretical prediction. At the point
where the interaction strength diverges, the energy and square of the wave
function for two distinguishable particles are the same as for a system of two
identical fermions. This is referred to as fermionization. We have observed
this phenomenon by directly comparing two distinguishable fermions with
diverging interaction strength with two identical fermions in the same
potential. We observe good agreement between experiment and theory. By adding
one or more particles our system can be used as a quantum simulator for more
complex few-body systems where no theoretical solution is available
Survey of Beam Optics Solutions for the MLS Lattice
The Metrology Light Source MLS is an electron storage ring containing 24 quadrupole magnets which can be powered individually. Fully exploring the capabilities of the machine optics by tracking or experiment would be very time consuming. Therefore the quadrupoles were combined in five families and a numerical brute force approach was used to scan for areas of stable solutions in the scope of linear beam optics. In order to get information on the expected beam lifetimes for each generated optics, a model for the Touschek lifetime was implemented. Simulation results as well as experimental tests of selected optics will be presente
THz Bursting Thresholds Measured at the Metrology Light Source
At the Metrology Light Source MLS [1] owned by the Physikalisch Technische Bundesanstalt PTB the bunch length can be varied by more than two orders of magnitude [2]. The bunch length manipulation is achieved by varying different machine parameters, such as RF voltage amplitude up to 500 kV and the momentum compaction factor amp; 945; over three orders of magnitude. The subject of this article is the measurement of THz bursting thresholds at the MLS for different bunch length
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