225 research outputs found
On the chromatic roots of generalized theta graphs
The generalized theta graph \Theta_{s_1,...,s_k} consists of a pair of
endvertices joined by k internally disjoint paths of lengths s_1,...,s_k \ge 1.
We prove that the roots of the chromatic polynomial $pi(\Theta_{s_1,...,s_k},z)
of a k-ary generalized theta graph all lie in the disc |z-1| \le [1 + o(1)]
k/\log k, uniformly in the path lengths s_i. Moreover, we prove that
\Theta_{2,...,2} \simeq K_{2,k} indeed has a chromatic root of modulus [1 +
o(1)] k/\log k. Finally, for k \le 8 we prove that the generalized theta graph
with a chromatic root that maximizes |z-1| is the one with all path lengths
equal to 2; we conjecture that this holds for all k.Comment: LaTex2e, 25 pages including 2 figure
Confusions in orbivirus protein classification
An extensive comparative analysis of orbivirus genomes revealed four cases of unclear numeration and protein designation, due to confused reference to protein size or segment size by which they are encoded. A concise nomenclature based on type species, sequence homology and functional characteristics independent of segment or protein size is suggested
World-line Quantisation of a Reciprocally Invariant System
We present the world-line quantisation of a system invariant under the
symmetries of reciprocal relativity (pseudo-unitary transformations on ``phase
space coordinates" which preserve the Minkowski
metric and the symplectic form, and global shifts in these coordinates,
together with coordinate dependent transformations of an additional compact
phase coordinate, ). The action is that of free motion over the
corresponding Weyl-Heisenberg group. Imposition of the first class constraint,
the generator of local time reparametrisations, on physical states enforces
identification of the world-line cosmological constant with a fixed value of
the quadratic Casimir of the quaplectic symmetry group , the semi-direct product of the pseudo-unitary group with
the Weyl-Heisenberg group (the central extension of the global translation
group, with central extension associated to the phase variable ).
The spacetime spectrum of physical states is identified. Even though for an
appropriate range of values the restriction enforced by the cosmological
constant projects out negative norm states from the physical spectrum, leaving
over spin zero states only, the mass-squared spectrum is continuous over the
entire real line and thus includes a tachyonic branch as well
Perturbative Hamiltonian constraints for higher order theories
We present a method for constructing a consistent low energy canonical
formalism for higher order time-derivative theories, extending the Dirac method
to include perturbative Hamiltonian constraints. We apply it to two
paradigmatic examples: the Pais-Uhlenbeck oscillator and the Bernard-Duncan
scalar field. We also compare the results, both at the classical and quantum
level, with the ones corresponding to a direct perturbative construction
applied to the exact higher order theory. This comparison highligths the
soundness of the present formalism.Comment: 26 pages, 4 figures; review section shortened and appendices change
A Seesaw Mechanism in the Higgs Sector
In this letter we revisit the seesaw Higgs mechanism. We show how a seesaw
mechanism in a two Higgs doublets model can trigger the electroweak symmetry
breaking if at least one of the eigenvalues of the squared mass matrix is
negative. We then consider two special cases of interest. In the decoupling
scenario, there is only one scalar degree of freedom in the low energy regime.
In the degenerate scenario, all five degrees of freedom are in the low energy
regime and will lead to observables effects at the LHC. Furthermore, in that
scenario, it is possible to impose a discrete symmetry between the doublets
that makes the extra neutral degrees of freedom stable. These are thus viable
dark matter candidates. We find an interesting relation between the electroweak
symmetry breaking mechanism and dark matter.Comment: 10 page
Genetic characterization of Bhanja virus and Palma virus, two tick-borne phleboviruses
The genomes of Bhanja virus (BHAV) and Palma virus (PALV) two tick-borne viruses hitherto grouped into the Bhanja virus antigenic complex of the Bunyaviridae were determined by pyrosequencing. Phylogenetic analysis groups all three segments of BHAV and PALV into a distinct clade of tick-borne phleboviruses together with the newly described severe fever with thrombocytopenia syndrome virus and Uukuniemi virus. The terminal signature sequences which are signatures for taxonomic grouping and important for virus replication and RNA transcription show marked differences in the L- and S-segments
Development of a Flow-Trough Microarray based Reverse Transcriptase Multiplex Ligation-Dependent Probe Amplification Assay for the Detection of European Bunyaviruses
It is suspected that apart from tick-borne encephalitis virus several additional European Arboviruses such as the sandfly borne Toscana virus, sandfly fever Sicilian virus and sandfly fever Naples virus, mosquito-borne Tahyna virus, Inkoo virus, Batai virus and tick-borne Uukuniemi virus cause aseptic meningo-encephalitis or febrile disease in Europe. Currently, the microarray technology is developing rapidly and there are many efforts to apply it to infectious diseases diagnostics. In order to arrive at an assay system useful for high throughput analysis of samples from aseptic meningo-encephalitis cases the authors developed a combined multiplex ligation-dependent probe amplification and flow-through microarray assay for the detection of European Bunyaviruses. These results show that this combined assay indeed is highly sensitive, and specific for the accurate detection of multiple viruses
Genetic Analysis of Floral Symmetry in Van Gogh's Sunflowers Reveals Independent Recruitment of CYCLOIDEA Genes in the Asteraceae
The genetic basis of floral symmetry is a topic of great interest because of its effect on pollinator behavior and, consequently, plant diversification. The Asteraceae, which is the largest family of flowering plants, is an ideal system in which to study this trait, as many species within the family exhibit a compound inflorescence containing both bilaterally symmetric (i.e., zygomorphic) and radially symmetric (i.e., actinomorphic) florets. In sunflower and related species, the inflorescence is composed of a single whorl of ray florets surrounding multiple whorls of disc florets. We show that in double-flowered (dbl) sunflower mutants (in which disc florets develop bilateral symmetry), such as those captured by Vincent van Gogh in his famous nineteenth-century sunflower paintings, an insertion into the promoter region of a CYCLOIDEA (CYC)-like gene (HaCYC2c) that is normally expressed specifically in WT rays is instead expressed throughout the inflorescence, presumably resulting in the observed loss of actinomorphy. This same gene is mutated in two independent tubular-rayed (tub) mutants, though these mutations involve apparently recent transposon insertions, resulting in little or no expression and radialization of the normally zygomorphic ray florets. Interestingly, a phylogenetic analysis of CYC-like genes from across the family suggests that different paralogs of this fascinating gene family have been independently recruited to specify zygomorphy in different species within the Asteraceae
Molecular phylogenetics reveal multiple tertiary vicariance origins of the African rain forest trees
Background - Tropical rain forests are the most diverse terrestrial ecosystems on the planet. How this diversity evolved remains largely unexplained. In Africa, rain forests are situated in two geographically isolated regions: the West-Central Guineo-Congolian region and the coastal and montane regions of East Africa. These regions have strong floristic affinities with each other, suggesting a former connection via an Eocene pan-African rain forest. High levels of endemism observed in both regions have been hypothesized to be the result of either 1) a single break-up followed by a long isolation or 2) multiple fragmentation and reconnection since the Oligocene. To test these hypotheses the evolutionary history of endemic taxa within a rain forest restricted African lineage of the plant family Annonaceae was studied. Molecular phylogenies and divergence dates were estimated using a Bayesian relaxed uncorrelated molecular clock assumption accounting for both calibration and phylogenetic uncertainties. Results - Our results provide strong evidence that East African endemic lineages of Annonaceae have multiple origins dated to significantly different times spanning the Oligocene and Miocene epochs. Moreover, these successive origins (c. 33, 16 and 8 million years ¿ Myr) coincide with known periods of aridification and geological activity in Africa that would have recurrently isolated the Guineo-Congolian rain forest from the East African one. All East African taxa were found to have diversified prior to Pleistocene times. Conclusion - Molecular phylogenetic dating analyses of this large pan-African clade of Annonaceae unravels an interesting pattern of diversification for rain forest restricted trees co-occurring in West/Central and East African rain forests. Our results suggest that repeated reconnections between the West/Central and East African rain forest blocks allowed for biotic exchange while the break-ups induced speciation via vicariance, enhancing the levels of endemicity. These results provide an explanation for present day distribution patterns and origins of endemicity for African rain forest trees. Moreover, given the pre-Pleistocene origins of all the studied endemic East African genera and species, these results also offer important insights for setting conservation priorities in these highly diversified but threatene
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