3,583 research outputs found
Divergences in QED on a Graph
We consider a model of quantum electrodynamics (QED) on a graph. The one-loop
divergences in the model are investigated by use of the background field
method.Comment: 14 pages, no figures, RevTeX4. References and typos adde
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Food System Transformation: Integrating a Political-Economy and Social-Ecological Approach to Regime Shifts.
Sustainably achieving the goal of global food security is one of the greatest challenges of the 21st century. The current food system is failing to meet the needs of people, and at the same time, is having far-reaching impacts on the environment and undermining human well-being in other important ways. It is increasingly apparent that a deep transformation in the way we produce and consume food is needed in order to ensure a more just and sustainable future. This paper uses the concept of regime shifts to understand key drivers and innovations underlying past disruptions in the food system and to explore how they may help us think about desirable future changes and how we might leverage them. We combine two perspectives on regime shifts-one derived from natural sciences and the other from social sciences-to propose an interpretation of food regimes that draws on innovation theory. We use this conceptualization to discuss three examples of innovations that we argue helped enable critical regime shifts in the global food system in the past: the Haber-Bosch process of nitrogen fixation, the rise of the supermarket, and the call for more transparency in the food system to reconnect consumers with their food. This paper concludes with an exploration of why this combination of conceptual understandings is important across the Global North/ Global South divide, and proposes a new sustainability regime where transformative change is spearheaded by a variety of social-ecological innovations
Some Exact Results on the Potts Model Partition Function in a Magnetic Field
We consider the Potts model in a magnetic field on an arbitrary graph .
Using a formula of F. Y. Wu for the partition function of this model as a
sum over spanning subgraphs of , we prove some properties of concerning
factorization, monotonicity, and zeros. A generalization of the Tutte
polynomial is presented that corresponds to this partition function. In this
context we formulate and discuss two weighted graph-coloring problems. We also
give a general structural result for for cyclic strip graphs.Comment: 5 pages, late
Aggregation and biofilm formation of bacteria isolated from domestic drinking water
The objective of this study was to investigate the autoaggregation, coaggregation and biofilm
formation of four bacteria namely Sphingobium, Xenophilus, Methylobacterium and Rhodococcus
isolated from drinking water. Auto and coaggregation studies were performed by both qualitative
(DAPI staining) and semi-quantitative (visual coaggregation) methods and biofilms produced by either
pure or dual-cultures were quantified by crystal violet method. Results from the semi-quantitative
visual aggregation method did not show any immediate auto or coaggregation, which was confirmed
by the 40
,6 diamidino-2-phenylindole (DAPI) staining method. However, after 2 hours,
Methylobacterium showed the highest autoaggregation of all four isolates. The Methylobacterium
combinations showed highest coaggregation between dual species at extended period of times (72
hours). Biofilm formation by pure cultures was negligible in comparison to the quantity of biofilm
produced by dual-species biofilms. The overall results show that coaggregation of bacteria isolated
from drinking water was mediated by species-specific and time-dependent interactions with a
synergistic type of biofilm formation. The results of this study are therefore a useful step in assisting
the development of potential control strategies by identifying specific bacteria that promote
aggregation or biofilm formation in drinking water distribution systems
Ground State Entropy of Potts Antiferromagnets on Cyclic Polygon Chain Graphs
We present exact calculations of chromatic polynomials for families of cyclic
graphs consisting of linked polygons, where the polygons may be adjacent or
separated by a given number of bonds. From these we calculate the (exponential
of the) ground state entropy, , for the q-state Potts model on these graphs
in the limit of infinitely many vertices. A number of properties are proved
concerning the continuous locus, , of nonanalyticities in . Our
results provide further evidence for a general rule concerning the maximal
region in the complex q plane to which one can analytically continue from the
physical interval where .Comment: 27 pages, Latex, 17 figs. J. Phys. A, in pres
Exact T=0 Partition Functions for Potts Antiferromagnets on Sections of the Simple Cubic Lattice
We present exact solutions for the zero-temperature partition function of the
-state Potts antiferromagnet (equivalently, the chromatic polynomial ) on
tube sections of the simple cubic lattice of fixed transverse size and arbitrarily great length , for sizes and and boundary conditions (a) and (b)
, where () denote free (periodic) boundary
conditions. In the limit of infinite-length, , we calculate the
resultant ground state degeneracy per site (= exponent of the ground-state
entropy). Generalizing from to , we determine
the analytic structure of and the related singular locus which
is the continuous accumulation set of zeros of the chromatic polynomial. For
the limit of a given family of lattice sections, is
analytic for real down to a value . We determine the values of
for the lattice sections considered and address the question of the value of
for a -dimensional Cartesian lattice. Analogous results are presented
for a tube of arbitrarily great length whose transverse cross section is formed
from the complete bipartite graph .Comment: 28 pages, latex, six postscript figures, two Mathematica file
Families of Graphs with W_r({G},q) Functions That Are Nonanalytic at 1/q=0
Denoting as the chromatic polynomial for coloring an -vertex
graph with colors, and considering the limiting function , a fundamental question in graph theory is the
following: is analytic or not at the origin
of the plane? (where the complex generalization of is assumed). This
question is also relevant in statistical mechanics because
, where is the ground state entropy of the
-state Potts antiferromagnet on the lattice graph , and the
analyticity of at is necessary for the large- series
expansions of . Although is analytic at for many
, there are some for which it is not; for these, has no
large- series expansion. It is important to understand the reason for this
nonanalyticity. Here we give a general condition that determines whether or not
a particular is analytic at and explains the
nonanalyticity where it occurs. We also construct infinite families of graphs
with functions that are non-analytic at and investigate the
properties of these functions. Our results are consistent with the conjecture
that a sufficient condition for to be analytic at is
that is a regular lattice graph . (This is known not to be a
necessary condition).Comment: 22 pages, Revtex, 4 encapsulated postscript figures, to appear in
Phys. Rev.
Ground State Entropy of Potts Antiferromagnets: Bounds, Series, and Monte Carlo Measurements
We report several results concerning , the
exponent of the ground state entropy of the Potts antiferromagnet on a lattice
. First, we improve our previous rigorous lower bound on for
the honeycomb (hc) lattice and find that it is extremely accurate; it agrees to
the first eleven terms with the large- series for . Second, we
investigate the heteropolygonal Archimedean lattice, derive a
rigorous lower bound, on , and calculate the large- series
for this function to where . Remarkably, these agree
exactly to all thirteen terms calculated. We also report Monte Carlo
measurements, and find that these are very close to our lower bound and series.
Third, we study the effect of non-nearest-neighbor couplings, focusing on the
square lattice with next-nearest-neighbor bonds.Comment: 13 pages, Latex, to appear in Phys. Rev.
Thermal X-rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars
Abridged) We model the X-ray properties of millisecond pulsars (MSPs) by
considering hot spot emission from a weakly magnetized rotating neutron star
(NS) covered by an optically-thick hydrogen atmosphere. We investigate the
limitations of using the thermal X-ray pulse profiles of MSPs to constrain the
mass-to-radius () ratio of the underlying NS. The accuracy is strongly
dependent on the viewing angle and magnetic inclination. For certain systems,
the accuracy is ultimately limited only by photon statistics implying that
future X-ray observatories could, in principle, achieve constraints on
and hence the NS equation of state to better than 5%. We demonstrate that
valuable information regarding the basic properties of the NS can be extracted
even from X-ray data of fairly limited photon statistics through modeling of
archival spectroscopic and timing observations of the nearby isolated PSRs
J0030+0451 and J2124--3358. The X-ray emission from these pulsars is consistent
with the presence of a hydrogen atmosphere and a dipolar magnetic field
configuration, in agreement with previous findings for PSR J0437--4715. For
both MSPs, the favorable geometry allows us to place interesting limits on the
allowed of NSs. Assuming 1.4 M, the stellar radius is
constrained to be km and km (68% confidence) for PSRs
J0030+0451 and J2124--3358, respectively. We explore the prospects of using
future observatories such as \textit{Constellation-X} and \textit{XEUS} to
conduct blind X-ray timing searches for MSPs not detectable at radio
wavelengths due to unfavorable viewing geometry. Using the observational
constraints on the pulsar obliquities we are also able to place strong
constraints on the magnetic field evolution model proposed by Ruderman.Comment: 9 pages, 7 figures, published in the Astrophysical Journal (Volume
689, Issue 1, pp. 407-415
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