6,015 research outputs found
Quasi-Monte Carlo Algorithms (not only) for Graphics Software
Quasi-Monte Carlo methods have become the industry standard in computer
graphics. For that purpose, efficient algorithms for low discrepancy sequences
are discussed. In addition, numerical pitfalls encountered in practice are
revealed. We then take a look at massively parallel quasi-Monte Carlo
integro-approximation for image synthesis by light transport simulation. Beyond
superior uniformity, low discrepancy points may be optimized with respect to
additional criteria, such as noise characteristics at low sampling rates or the
quality of low-dimensional projections
Discrete Optimization for Interpretable Study Populations and Randomization Inference in an Observational Study of Severe Sepsis Mortality
Motivated by an observational study of the effect of hospital ward versus
intensive care unit admission on severe sepsis mortality, we develop methods to
address two common problems in observational studies: (1) when there is a lack
of covariate overlap between the treated and control groups, how to define an
interpretable study population wherein inference can be conducted without
extrapolating with respect to important variables; and (2) how to use
randomization inference to form confidence intervals for the average treatment
effect with binary outcomes. Our solution to problem (1) incorporates existing
suggestions in the literature while yielding a study population that is easily
understood in terms of the covariates themselves, and can be solved using an
efficient branch-and-bound algorithm. We address problem (2) by solving a
linear integer program to utilize the worst case variance of the average
treatment effect among values for unobserved potential outcomes that are
compatible with the null hypothesis. Our analysis finds no evidence for a
difference between the sixty day mortality rates if all individuals were
admitted to the ICU and if all patients were admitted to the hospital ward
among less severely ill patients and among patients with cryptic septic shock.
We implement our methodology in R, providing scripts in the supplementary
material
Calabi-Yau threefolds with large h^{2, 1}
We carry out a systematic analysis of Calabi-Yau threefolds that are
elliptically fibered with section ("EFS") and have a large Hodge number h^{2,
1}. EFS Calabi-Yau threefolds live in a single connected space, with regions of
moduli space associated with different topologies connected through transitions
that can be understood in terms of singular Weierstrass models. We determine
the complete set of such threefolds that have h^{2, 1} >= 350 by tuning
coefficients in Weierstrass models over Hirzebruch surfaces. The resulting set
of Hodge numbers includes those of all known Calabi-Yau threefolds with h^{2,
1} >= 350, as well as three apparently new Calabi-Yau threefolds. We speculate
that there are no other Calabi-Yau threefolds (elliptically fibered or not)
with Hodge numbers that exceed this bound. We summarize the theoretical and
practical obstacles to a complete enumeration of all possible EFS Calabi-Yau
threefolds and fourfolds, including those with small Hodge numbers, using this
approach.Comment: 44 pages, 5 tables, 5 figures; v2: minor corrections; v3: minor
corrections, moved figure; v4: typo in Table 2 correcte
Fusion multiplicities as polytope volumes: N-point and higher-genus su(2) fusion
We present the first polytope volume formulas for the multiplicities of
affine fusion, the fusion in Wess-Zumino-Witten conformal field theories, for
example. Thus, we characterise fusion multiplicities as discretised volumes of
certain convex polytopes, and write them explicitly as multiple sums measuring
those volumes. We focus on su(2), but discuss higher-point (N>3) and
higher-genus fusion in a general way. The method follows that of our previous
work on tensor product multiplicities, and so is based on the concepts of
generalised Berenstein-Zelevinsky diagrams, and virtual couplings. As a
by-product, we also determine necessary and sufficient conditions for
non-vanishing higher-point fusion multiplicities. In the limit of large level,
these inequalities reduce to very simple non-vanishing conditions for the
corresponding tensor product multiplicities. Finally, we find the minimum level
at which the higher-point fusion and tensor product multiplicities coincide.Comment: 14 pages, LaTeX, version to be publishe
- …