11,308 research outputs found
Rational Conformal Field Theories and Complex Multiplication
We study the geometric interpretation of two dimensional rational conformal
field theories, corresponding to sigma models on Calabi-Yau manifolds. We
perform a detailed study of RCFT's corresponding to T^2 target and identify the
Cardy branes with geometric branes. The T^2's leading to RCFT's admit ``complex
multiplication'' which characterizes Cardy branes as specific D0-branes. We
propose a condition for the conformal sigma model to be RCFT for arbitrary
Calabi-Yau n-folds, which agrees with the known cases. Together with recent
conjectures by mathematicians it appears that rational conformal theories are
not dense in the space of all conformal theories, and sometimes appear to be
finite in number for Calabi-Yau n-folds for n>2. RCFT's on K3 may be dense. We
speculate about the meaning of these special points in the moduli spaces of
Calabi-Yau n-folds in connection with freezing geometric moduli.Comment: 39 pages, 6 figures, harvmac; references adde
Random graph ensembles with many short loops
Networks observed in the real world often have many short loops. This
violates the tree-like assumption that underpins the majority of random graph
models and most of the methods used for their analysis. In this paper we sketch
possible research routes to be explored in order to make progress on networks
with many short loops, involving old and new random graph models and ideas for
novel mathematical methods. We do not present conclusive solutions of problems,
but aim to encourage and stimulate new activity and in what we believe to be an
important but under-exposed area of research. We discuss in more detail the
Strauss model, which can be seen as the `harmonic oscillator' of `loopy' random
graphs, and a recent exactly solvable immunological model that involves random
graphs with extensively many cliques and short loops.Comment: 18 pages, 10 figures,Mathematical Modelling of Complex Systems (Paris
2013) conferenc
On Frobenius incidence varieties of linear subspaces over finite fields
We define Frobenius incidence varieties by means of the incidence relation of
Frobenius images of linear subspaces in a fixed vector space over a finite
field, and investigate their properties such as supersingularity, Betti numbers
and unirationality. These varieties are variants of the Deligne-Lusztig
varieties. We then study the lattices associated with algebraic cycles on them.
We obtain a positive-definite lattice of rank 84 that yields a dense sphere
packing from a 4-dimensional Frobenius incidence variety in characteristic 2.Comment: 24 pages, no figures; Introduction is changed. New references are
adde
Geometric Exponents of Dilute Logarithmic Minimal Models
The fractal dimensions of the hull, the external perimeter and of the red
bonds are measured through Monte Carlo simulations for dilute minimal models,
and compared with predictions from conformal field theory and SLE methods. The
dilute models used are those first introduced by Nienhuis. Their loop fugacity
is beta = -2cos(pi/barkappa}) where the parameter barkappa is linked to their
description through conformal loop ensembles. It is also linked to conformal
field theories through their central charges c = 13 - 6(barkappa +
barkappa^{-1}) and, for the minimal models of interest here, barkappa = p/p'
where p and p' are two coprime integers. The geometric exponents of the hull
and external perimeter are studied for the pairs (p,p') = (1,1), (2,3), (3,4),
(4,5), (5,6), (5,7), and that of the red bonds for (p,p') = (3,4). Monte Carlo
upgrades are proposed for these models as well as several techniques to improve
their speeds. The measured fractal dimensions are obtained by extrapolation on
the lattice size H,V -> infinity. The extrapolating curves have large slopes;
despite these, the measured dimensions coincide with theoretical predictions up
to three or four digits. In some cases, the theoretical values lie slightly
outside the confidence intervals; explanations of these small discrepancies are
proposed.Comment: 41 pages, 32 figures, added reference
- …