13,840 research outputs found

### Two-dimensional conformal field theory and the butterfly effect

We study chaotic dynamics in two-dimensional conformal field theory through
out-of-time order thermal correlators of the form $\langle W(t)VW(t)V\rangle$.
We reproduce bulk calculations similar to those of [1], by studying the large
$c$ Virasoro identity block. The contribution of this block to the above
correlation function begins to decrease exponentially after a delay of $\sim
t_* - \frac{\beta}{2\pi}\log \beta^2E_w E_v$, where $t_*$ is the scrambling
time $\frac{\beta}{2\pi}\log c$, and $E_w,E_v$ are the energy scales of the
$W,V$ operators.Comment: v1: 14 pages plus appendices, 2 figures. v2: references updated and
minor changes to the text. v3: minor error corrected in Appendix B, but the
conclusion is unchange

### Summary Account of the Carolina Parakeet in Arkansas

The extinct Carolina Parakeet (Conuropsis carolinensis) once was part of the Arkansas avifauna. The first two reports of the species in what is now Arkansas were made in 1673 and 1718 by early French explorers. The remaining records are from the 1800s when parakeets were found in nearly all parts of the state, often in abundance. The last literature reference for the species still definitely occurring in Arkansas pertains to birds present in the summer of 1885 along the White River at Newpor

### Neural Representations for Sensory-Motor Control, III: Learning a Body-Centered Representation of 3-D Target Position

A neural model is described of how the brain may autonomously learn a body-centered representation of 3-D target position by combining information about retinal target position, eye position, and head position in real time. Such a body-centered spatial representation enables accurate movement commands to the limbs to be generated despite changes in the spatial relationships between the eyes, head, body, and limbs through time. The model learns a vector representation--otherwise known as a parcellated distributed representation--of target vergence with respect to the two eyes, and of the horizontal and vertical spherical angles of the target with respect to a cyclopean egocenter. Such a vergence-spherical representation has been reported in the caudal midbrain and medulla of the frog, as well as in psychophysical movement studies in humans. A head-centered vergence-spherical representation of foveated target position can be generated by two stages of opponent processing that combine corollary discharges of outflow movement signals to the two eyes. Sums and differences of opponent signals define angular and vergence coordinates, respectively. The head-centered representation interacts with a binocular visual representation of non-foveated target position to learn a visuomotor representation of both foveated and non-foveated target position that is capable of commanding yoked eye movementes. This head-centered vector representation also interacts with representations of neck movement commands to learn a body-centered estimate of target position that is capable of commanding coordinated arm movements. Learning occurs during head movements made while gaze remains fixed on a foveated target. An initial estimate is stored and a VOR-mediated gating signal prevents the stored estimate from being reset during a gaze-maintaining head movement. As the head moves, new estimates arc compared with the stored estimate to compute difference vectors which act as error signals that drive the learning process, as well as control the on-line merging of multimodal information.Air Force Office of Scientific Research (F49620-92-J-0499); National Science Foundation (IRI -87-16960, IRI-90-24877); Office of Naval Research (N00014-92-J-l309

### Legality and venture governance around the world

We analyze governance with a dataset on investments of venture capitalists in 3848 portfolio firms in 39 countries from North and South America, Europe and Asia spanning 1971-2003. We find that cross-country differences in Legality have a significant impact on the governance structure of investments in the VC industry: better laws facilitate faster deal screening and deal origination, a higher probability of syndication and a lower probability of potentially harmful co-investment, and facilitate board representation of the investor. We also show better laws reduce the probability that the investor requires periodic cash flows prior to exit, which is in conjunction with an increased probability of investment in high-tech companies. Klassifikation: G24, G31, G32

### Localized shocks

We study products of precursors of spatially local operators,
$W_{x_{n}}(t_{n}) ... W_{x_1}(t_1)$, where $W_x(t) = e^{-iHt} W_x e^{iHt}$.
Using chaotic spin-chain numerics and gauge/gravity duality, we show that a
single precursor fills a spatial region that grows linearly in $t$. In a
lattice system, products of such operators can be represented using tensor
networks. In gauge/gravity duality, they are related to Einstein-Rosen bridges
supported by localized shock waves. We find a geometrical correspondence
between these two descriptions, generalizing earlier work in the spatially
homogeneous case.Comment: 23 pages plus appendices, 12 figures. v2: minor error in Appendix B
corrected. v3: figure added to the introduction comparing the butterfly
effect cone with the standard light con

### THE INFLUENCE OF TECHNOLOGICAL PROGRESS ON THE LONG RUN FARM LEVEL ECONOMICS OF SOIL CONSERVATION

The complementary interaction between topsoil depth and technical progress for winter wheat in the Palouse region was found to strengthen the long run payoff to conservation tillage. Nonetheless, conservation tillage was found to be competitive with conventional tillage only if its current yield disadvantages were eliminated. Conservation tillage was relatively more competitive on shallower topsoils and for longer planning horizons. Short-term subsidies coupled with research directed towards reducing the cost and yield disadvantages of conservation tillage in the Palouse were advocated to maintain long-term soil productivity.Land Economics/Use, Research and Development/Tech Change/Emerging Technologies,

### Short Proofs for Cut-and-Paste Sorting of Permutations

We consider the problem of determining the maximum number of moves required
to sort a permutation of $[n]$ using cut-and-paste operations, in which a
segment is cut out and then pasted into the remaining string, possibly
reversed. We give short proofs that every permutation of $[n]$ can be
transformed to the identity in at most \flr{2n/3} such moves and that some
permutations require at least \flr{n/2} moves.Comment: 7 pages, 2 figure

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