273,114 research outputs found
Comparison of non-crossing perturbative approach and generalized projection method for strongly coupled spin-fermion systems at low doping
We analyze the two-dimensional spin-fermion model in the strong coupling
regime relevant to underdoped cuprates. We recall the set of general sumrules
that relate moments of spectral density and the imaginary part of fermion
self-energy with static correlation functions. We show that two-pole
approximation of projection method satisfies the sumrules for first four
moments of spectral density and gives an exact upper bound for quasiparticle
energy near the band bottom. We prove that non-crossing approximation that is
often made in perturbative consideration of the model violates the sumrule for
third moment of spectral density. This leads to wrong position of lowest
quasiparticle band. On the other hand, the projection method is inadequate in
weak coupling limit because of approximate treatment of kinetic energy term. We
propose a generalization of projection method that overcomes this default and
give the fermion self-energy that correctly behaves both in weak and strong
coupling limits.Comment: 9 pages, 4 EPS figures, RevTe
Perfect and near perfect adaptation in a model of bacterial chemotaxis
The signaling apparatus mediating bacterial chemotaxis can adapt to a wide
range of persistent external stimuli. In many cases, the bacterial activity
returns to its pre-stimulus level exactly and this "perfect adaptability" is
robust against variations in various chemotaxis protein concentrations. We
model the bacterial chemotaxis signaling pathway, from ligand binding to CheY
phosphorylation. By solving the steady-state equations of the model
analytically, we derive a full set of conditions for the system to achieve
perfect adaptation. The conditions related to the phosphorylation part of the
pathway are discovered for the first time, while other conditions are
generalization of the ones found in previous works. Sensitivity of the perfect
adaptation is evaluated by perturbing these conditions. We find that, even in
the absence of some of the perfect adaptation conditions, adaptation can be
achieved with near perfect precision as a result of the separation of scales in
both chemotaxis protein concentrations and reaction rates, or specific
properties of the receptor distribution in different methylation states. Since
near perfect adaptation can be found in much larger regions of the parameter
space than that defined by the perfect adaptation conditions, their existence
is essential to understand robustness in bacterial chemotaxis.Comment: 16 pages, 9 figure
Exact Dynamics of the SU(K) Haldane-Shastry Model
The dynamical structure factor of the SU(K) (K=2,3,4)
Haldane-Shastry model is derived exactly at zero temperature for arbitrary size
of the system. The result is interpreted in terms of free quasi-particles which
are generalization of spinons in the SU(2) case; the excited states relevant to
consist of K quasi-particles each of which is characterized by a
set of K-1 quantum numbers. Near the boundaries of the region where
is nonzero, shows the power-law singularity. It is
found that the divergent singularity occurs only in the lowest edges starting
from toward positive and negative q. The analytic result
is checked numerically for finite systems via exact diagonalization and
recursion methods.Comment: 35 pages, 3 figures, youngtab.sty (version 1.1
Online Influence Maximization under Independent Cascade Model with Semi-Bandit Feedback
We study the online influence maximization problem in social networks under
the independent cascade model. Specifically, we aim to learn the set of "best
influencers" in a social network online while repeatedly interacting with it.
We address the challenges of (i) combinatorial action space, since the number
of feasible influencer sets grows exponentially with the maximum number of
influencers, and (ii) limited feedback, since only the influenced portion of
the network is observed. Under a stochastic semi-bandit feedback, we propose
and analyze IMLinUCB, a computationally efficient UCB-based algorithm. Our
bounds on the cumulative regret are polynomial in all quantities of interest,
achieve near-optimal dependence on the number of interactions and reflect the
topology of the network and the activation probabilities of its edges, thereby
giving insights on the problem complexity. To the best of our knowledge, these
are the first such results. Our experiments show that in several representative
graph topologies, the regret of IMLinUCB scales as suggested by our upper
bounds. IMLinUCB permits linear generalization and thus is both statistically
and computationally suitable for large-scale problems. Our experiments also
show that IMLinUCB with linear generalization can lead to low regret in
real-world online influence maximization.Comment: Compared with the previous version, this version has fixed a mistake.
This version is also consistent with the NIPS camera-ready versio
Semidefinite programming characterization and spectral adversary method for quantum complexity with noncommuting unitary queries
Generalizing earlier work characterizing the quantum query complexity of
computing a function of an unknown classical ``black box'' function drawn from
some set of such black box functions, we investigate a more general quantum
query model in which the goal is to compute functions of N by N ``black box''
unitary matrices drawn from a set of such matrices, a problem with applications
to determining properties of quantum physical systems. We characterize the
existence of an algorithm for such a query problem, with given error and number
of queries, as equivalent to the feasibility of a certain set of semidefinite
programming constraints, or equivalently the infeasibility of a dual of these
constraints, which we construct. Relaxing the primal constraints to correspond
to mere pairwise near-orthogonality of the final states of a quantum computer,
conditional on black-box inputs having distinct function values, rather than
bounded-error determinability of the function value via a single measurement on
the output states, we obtain a relaxed primal program the feasibility of whose
dual still implies the nonexistence of a quantum algorithm. We use this to
obtain a generalization, to our not-necessarily-commutative setting, of the
``spectral adversary method'' for quantum query lower bounds.Comment: Dagstuhl Seminar Proceedings 06391, "Algorithms and Complexity for
Continuous Problems," ed. S. Dahlke, K. Ritter, I. H. Sloan, J. F. Traub
(2006), available electronically at
http://drops.dagstuhl.de/portals/index.php?semnr=0639
On the spectra of scalar mesons from HQCD models
We determine the holographic spectra of scalar mesons from the fluctuations
of the embedding of flavor D-brane probes in HQCD models. The models we
consider include a generalization of the Sakai Sugimoto model at zero
temperature and at the "high-temperature intermediate phase", where the system
is in a deconfining phase while admitting chiral symmetry breaking and a
non-critical 6d model at zero temperature. All these models are based on
backgrounds associated with near extremal N_c D4 branes and a set of N_f<<N_c
flavor probe branes that admit geometrical chiral symmetry breaking. We point
out that the spectra of these models include a 0^{--} branch which does not
show up in nature.
At zero temperature we found that the masses of the mesons M_n depend on the
"constituent quark mass" parameter m^c_q and on the excitation number n as
M_n^2 m^c_q and M^2_n n^{1.7} for the ten dimensional case and as M_n m^c_q and
M_n n^{0.75} in the non-critical case. At the high temperature intermediate
phase we detect a decrease of the masses of low spin mesons as a function of
the temperature similar to holographic vector mesons and to lattice
calculations.Comment: 22 pages, 12 figure
Relating Regularization and Generalization through the Intrinsic Dimension of Activations
Given a pair of models with similar training set performance, it is natural
to assume that the model that possesses simpler internal representations would
exhibit better generalization. In this work, we provide empirical evidence for
this intuition through an analysis of the intrinsic dimension (ID) of model
activations, which can be thought of as the minimal number of factors of
variation in the model's representation of the data. First, we show that common
regularization techniques uniformly decrease the last-layer ID (LLID) of
validation set activations for image classification models and show how this
strongly affects generalization performance. We also investigate how excessive
regularization decreases a model's ability to extract features from data in
earlier layers, leading to a negative effect on validation accuracy even while
LLID continues to decrease and training accuracy remains near-perfect. Finally,
we examine the LLID over the course of training of models that exhibit
grokking. We observe that well after training accuracy saturates, when models
``grok'' and validation accuracy suddenly improves from random to perfect,
there is a co-occurent sudden drop in LLID, thus providing more insight into
the dynamics of sudden generalization.Comment: NeurIPS 2022 OPT and HITY workshop
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