9,615 research outputs found
A Cluster Bootstrap for Two-Loop MHV Amplitudes
We apply a bootstrap procedure to two-loop MHV amplitudes in planar N=4
super-Yang-Mills theory. We argue that the mathematically most complicated part
(the coproduct component) of the n-particle amplitude is
uniquely determined by a simple cluster algebra property together with a few
physical constraints (dihedral symmetry, analytic structure, supersymmetry, and
well-defined collinear limits). We present a concise, closed-form expression
which manifests these properties for all n.Comment: v2: a few comments adde
Liquidity when it matters : QE and Tobinâs q
When financial markets freeze in fear, borrowing costs for solvent governments may fall towards zero in a flight to quality â but credit-worthy private borrowers can be
starved of external funding. In Kiyotaki and Moore (2008), where liquidity crisis is captured by the effective rationing of private credit, tightening credit constraints have
direct effects on investment. If prices are sticky, the effects on aggregate demand can be pronounced â as reported by FRBNY for the US economy using a calibrated
DSGE-style framework modified to include such frictions.
In such an environment, two factors stand out. First the recycling of credit flows by central banks can dramatically ease credit-rationing faced by private investors: this is
the rationale for Quantitative Easing. Second, revenue-neutral fiscal transfers aimed at would-be investors can have similar effects. We show these features in a stripped- down macro model of inter-temporal optimisation subject to credit constraints
An analytic result for the two-loop seven-point MHV amplitude in N=4 SYM
We describe a general algorithm which builds on several pieces of data
available in the literature to construct explicit analytic formulas for
two-loop MHV amplitudes in N=4 super-Yang-Mills theory. The non-classical part
of an amplitude is built from cluster polylogarithm functions; classical
polylogarithms with (negative) cluster X-coordinate arguments are added to
complete the symbol of the amplitude; beyond-the-symbol terms proportional to
are determined by comparison with the differential of the amplitude;
and the overall additive constant is fixed by the collinear limit. We present
an explicit formula for the seven-point amplitude as a sample
application.Comment: 17 pages, 1 figure; v2: minor changes; v3: added a section with some
background materia
Assertion and transparent self-knowledge
We argue that honesty in assertion requires non-empirical knowledge that what one asserts is what one believes. Our argument proceeds from the thought that to assert honestly, one must follow and not merely conform to the norm âAssert that p only if you believe that pâ. Furthermore, careful consideration of cases shows that the sort of doxastic self-knowledge required for following this norm cannot be acquired on the basis of observation, inference, or any other form of detection of oneâs own doxastic states. It is, as we put it, transparent rather than empirical self-knowledge
Optimal Dynamic Nonlinear Income Taxes with No Commitment
We wish to study optimal dynamic nonlinear income taxes. Do real world taxes share some of their features? What policy prescriptions can be made? We study a two period model, where the consumers and government each have separate budget constraints in the two periods, so income cannot be transferred between periods. Labor supply in both periods is chosen by the consumers. The government has memory, so taxes in the first period are a function of first period labor income, whereas taxes in the second period are a function of both first and second period labor income. The government cannot commit to future taxes. Time consistency is thus imposed as a requirement. The main results of the paper show that time consistent incentive compatible two period taxes involve separation of types in the first period and a differentiated lump sum tax in the second period, provided that the discount rate is high or utility is separable between labor and consumption. In the natural extension of the Diamond (1998) model with quasi-linear utility functions to two periods, an equivalence of dynamic and static optimal taxes is demonstrated, and a necessary condition for the top marginal tax rate on first period income is found.Optimal Income Taxation; Time Consistency; Incentive Compatibility; Sequential Information Revelation; Optimal Dynamic Taxation
Universal Reinforcement Learning Algorithms: Survey and Experiments
Many state-of-the-art reinforcement learning (RL) algorithms typically assume
that the environment is an ergodic Markov Decision Process (MDP). In contrast,
the field of universal reinforcement learning (URL) is concerned with
algorithms that make as few assumptions as possible about the environment. The
universal Bayesian agent AIXI and a family of related URL algorithms have been
developed in this setting. While numerous theoretical optimality results have
been proven for these agents, there has been no empirical investigation of
their behavior to date. We present a short and accessible survey of these URL
algorithms under a unified notation and framework, along with results of some
experiments that qualitatively illustrate some properties of the resulting
policies, and their relative performance on partially-observable gridworld
environments. We also present an open-source reference implementation of the
algorithms which we hope will facilitate further understanding of, and
experimentation with, these ideas.Comment: 8 pages, 6 figures, Twenty-sixth International Joint Conference on
Artificial Intelligence (IJCAI-17
Alignment of cryo-EM movies of individual particles by optimization of image translations
Direct detector device (DDD) cameras have revolutionized single particle
electron cryomicroscopy (cryo-EM). In addition to an improved camera detective
quantum efficiency, acquisition of DDD movies allows for correction of movement
of the specimen, due both to instabilities in the microscope specimen stage and
electron beam-induced movement. Unlike specimen stage drift, beam-induced
movement is not always homogeneous within an image. Local correlation in the
trajectories of nearby particles suggests that beam-induced motion is due to
deformation of the ice layer. Algorithms have already been described that can
correct movement for large regions of frames and for > 1 MDa protein particles.
Another algorithm allows individual < 1 MDa protein particle trajectories to be
estimated, but requires rolling averages to be calculated from frames and fits
linear trajectories for particles. Here we describe an algorithm that allows
for individual < 1 MDa particle images to be aligned without frame averaging or
linear trajectories. The algorithm maximizes the overall correlation of the
shifted frames with the sum of the shifted frames. The optimum in this single
objective function is found efficiently by making use of analytically
calculated derivatives of the function. To smooth estimates of particle
trajectories, rapid changes in particle positions between frames are penalized
in the objective function and weighted averaging of nearby trajectories ensures
local correlation in trajectories. This individual particle motion correction,
in combination with weighting of Fourier components to account for increasing
radiation damage in later frames, can be used to improve 3-D maps from single
particle cryo-EM.Comment: 11 pages, 4 figure
International Volatility and Economic Growth: The First Ten Years of The International Seminar on Macroeconomics
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