4,417 research outputs found
A Formal Framework for Speedup Learning from Problems and Solutions
Speedup learning seeks to improve the computational efficiency of problem
solving with experience. In this paper, we develop a formal framework for
learning efficient problem solving from random problems and their solutions. We
apply this framework to two different representations of learned knowledge,
namely control rules and macro-operators, and prove theorems that identify
sufficient conditions for learning in each representation. Our proofs are
constructive in that they are accompanied with learning algorithms. Our
framework captures both empirical and explanation-based speedup learning in a
unified fashion. We illustrate our framework with implementations in two
domains: symbolic integration and Eight Puzzle. This work integrates many
strands of experimental and theoretical work in machine learning, including
empirical learning of control rules, macro-operator learning, Explanation-Based
Learning (EBL), and Probably Approximately Correct (PAC) Learning.Comment: See http://www.jair.org/ for any accompanying file
Supermassive black hole formation during the assembly of pre-galactic discs
In this paper we discuss the evolution of gravitationally unstable
pre-galactic discs that result from the collapse of haloes at high redshift or so, which have not yet been enriched by metals. In cases where
molecular hydrogen formation is suppressed the discs are maintained at a
temperature of a few thousand degrees Kelvin. However, when molecular hydrogen
is present cooling can proceed down to a few hundred degrees Kelvin. Analogous
to the case of the larger scale proto-galactic discs, we assume that the
evolution of these discs is mainly driven by angular momentum redistribution
induced by the development of gravitational instabilities in the disc. We also
properly take into account the possibility of disc fragmentation. We thus show
that this simple model naturally predicts the formation of supermassive black
holes in the nuclei of such discs and provides a robust determination of their
mass distribution as a function of halo properties. We estimate that roughly 5%
of discs resulting from the collapse of haloes with
should host a massive black hole with a mass . We confirm our arguments with time-dependent calculations of the
evolution of the surface density and of the accretion rate in these primordial
discs. This mechanism offers an efficient way to form seed black holes at high
redshift. The predicted masses for our black hole seeds enable the comfortable
assembly of black holes powering the luminous quasars detected
by the Sloan Digital Sky Survey at for a concordance cosmology.
(abridged)Comment: 12 pages, 8 figures, submitted to MNRA
The Gamma Ray Burst Luminosity Function in the Light of the Swift 2-year Data
We compute the luminosity function (LF) and the formation rate of long gamma
ray bursts (GRBs) by fitting the observed differential peak flux distribution
obtained by the BATSE satellite in three different scenarios: i) GRBs follow
the cosmic star formation and their LF is constant in time; ii) GRBs follow the
cosmic star formation but the LF varies with redshift; iii) GRBs form
preferentially in low-metallicity environments. We find that the differential
peak flux number counts obtained by BATSE and by Swift can be reproduced using
the same LF and GRB formation rate, indicating that the two satellites are
observing the same GRB population. We then check the resulting redshift
distributions in the light of Swift 2-year data, focusing in particular on the
relatively large sample of GRBs detected at z>2.5. We show that models in which
GRBs trace the cosmic star formation and are described by a constant LF are
ruled out by the number of high-z Swift detections. This conclusion does not
depend on the redshift distribution of bursts that lack of optical
identification, nor on the existence of a decline in star formation rate at
z>2, nor on the adopted faint-end of the GRB LF. Swift observations can be
explained by assuming that the LF varies with redshift and/or that GRB
formation is limited to low-metallicity environments.Comment: 7 pages, 3 figures, ApJ Letter in pres
New observational Constraints on the Growth of the First Supermassive Black Holes
We constrain the total accreted mass density in supermassive black holes at
z>6, inferred via the upper limit derived from the integrated X-ray emission
from a sample of photometrically selected galaxy candidates. Studying galaxies
obtained from the deepest Hubble Space Telescope images combined with the
Chandra 4 Msec observations of the Chandra Deep Field South, we achieve the
most restrictive constraints on total black hole growth in the early Universe.
We estimate an accreted mass density <1000Mo Mpc^-3 at z~6, significantly lower
than the previous predictions from some existing models of early black hole
growth and earlier prior observations. These results place interesting
constraints on early black growth and mass assembly by accretion and imply one
or more of the following: (1) only a fraction of the luminous galaxies at this
epoch contain active black holes; (2) most black hole growth at early epochs
happens in dusty and/or less massive - as yet undetected - host galaxies; (3)
there is a significant fraction of low-z interlopers in the galaxy sample; (4)
early black hole growth is radiatively inefficient, heavily obscured and/or is
due to black hole mergers as opposed to accretion or (5) the bulk of the black
hole growth occurs at late times. All of these possibilities have important
implications for our understanding of high redshift seed formation models.Comment: ApJ Accepted, 10 pages, 7 figures, 1 table, in emulateapj forma
Redescription of the flat fish Pardachirus marmoratus (Lacepede) from Indian waters
The note describes Pardachirus marmoratus (Lacepede)
(Ordcr : Pleronectiformes, Family : Soleidae)occuring in the Gulf of Mannar along the Indian Coast
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