Screening Stringy Horizons

It has been argued recently that string theory effects qualitatively modify the effective black hole geometry experienced by modes with radial momentum of order $1/\sqrt{\alpha'}$. At tree level, these $\alpha'$-effects can be explicitly worked out in two-dimensional string theory, and have a natural explanation in the T-dual description as coming from the integration of the zero-mode of the linear dilaton, what yields a contribution that affects the scattering phase-shift in a peculiar manner. It has also been argued that the phase-shift modification has its origin in a region of the moduli space that does not belong to the exterior black hole geometry, leading to the conclusion that at high energy the physics of the problem is better described by the dual model. Here, we elaborate on this argument. We consider the contribution of worldsheet instantons in the 2D Euclidean black hole sigma-model and study its influence on the phase-shift at high energy.Comment: 14 page

Timelike duality, M′M'-theory and an exotic form of the Englert solution

Through timelike dualities, one can generate exotic versions of $M$-theory with different spacetime signatures. These are the $M^*$-theory with signature $(9,2,-)$, the $M'$-theory, with signature $(6,5,+)$ and the theories with reversed signatures $(1,10, -)$, $(2,9, +)$ and $(5,6, -)$. In $(s,t, \pm)$, $s$ is the number of space directions, $t$ the number of time directions, and $\pm$ refers to the sign of the kinetic term of the $3$ form. The only irreducible pseudo-riemannian manifolds admitting absolute parallelism are, besides Lie groups, the seven-sphere $S^7 \equiv SO(8)/SO(7)$ and its pseudo-riemannian version $S^{3,4} \equiv SO(4,4)/SO(3,4)$. [There is also the complexification $SO(8,\mathbb{C})/SO(7, \mathbb{C})$, but it is of dimension too high for our considerations.] The seven-sphere $S^7\equiv S^{7,0}$ has been found to play an important role in $11$-dimensional supergravity, both through the Freund-Rubin solution and the Englert solution that uses its remarkable parallelizability to turn on non trivial internal fluxes. The spacetime manifold is in both cases $AdS_4 \times S^7$. We show that $S^{3,4}$ enjoys a similar role in $M'$-theory and construct the exotic form $AdS_4 \times S^{3,4}$ of the Englert solution, with non zero internal fluxes turned on. There is no analogous solution in $M^*$-theory.Comment: 18 pages, v2: typos fixe

Tight Bounds for Maximal Identifiability of Failure Nodes in Boolean Network Tomography

We study maximal identifiability, a measure recently introduced in Boolean Network Tomography to characterize networks' capability to localize failure nodes in end-to-end path measurements. We prove tight upper and lower bounds on the maximal identifiability of failure nodes for specific classes of network topologies, such as trees and $d$-dimensional grids, in both directed and undirected cases. We prove that directed $d$-dimensional grids with support $n$ have maximal identifiability $d$ using $2d(n-1)+2$ monitors; and in the undirected case we show that $2d$ monitors suffice to get identifiability of $d-1$. We then study identifiability under embeddings: we establish relations between maximal identifiability, embeddability and graph dimension when network topologies are model as DAGs. Our results suggest the design of networks over $N$ nodes with maximal identifiability $\Omega(\log N)$ using $O(\log N)$ monitors and a heuristic to boost maximal identifiability on a given network by simulating $d$-dimensional grids. We provide positive evidence of this heuristic through data extracted by exact computation of maximal identifiability on examples of small real networks

Prediction of repair and maintenance costs of two-wheel tractors in Iran

Predicting of repair and maintenance (R&M) costs of tractors in any mechanized farm is necessary for owners and managers to obtain information on overall maintenance costs in order to control production cost. In this context a study was conducted to predict accumulated R&M costs (Y) of two-wheel drive (2WD) tractors based on total usage hours (X) in Misagh-e- Sabz Agribusiness Company in Iran. Recorded data from the company was used to determine regression models for predicting total R&M costs (as percentage of initial purchase price) based on total usage hours. The statistical results of the study indicated that in order to predict total R&M costs of 2WD tractors with based on usage hours of 2260 h or less the power regression model Y = 0.013 (X/100)1.677 with R2 = 0.976, and to predict total R&M costs of 2WD tractors with based on usage hours of 2260 hours or more the polynomial regression model Y = 0.004 (X/100)2 – 0.181 (X/100) + 4.373 with R2 = 0.998 is strongly suggested. Results of the study also indicated that total R&M costs predicted in this study were lower than those predicted in the previous studies. Cumulative life of 12000 hours, for instance, indicated an total R&M costs equivalent to 40.3% of initial purchase price. The most likely reasons that explain these differences between the predicted results in this study and those predicted in the previous studies probably can be attributed to new tractor technology for production during the last two decades and lower labor wages in Iran

Conformal field theories from deformations of theories with WnW_n symmetry

We construct a set of non-rational conformal field theories that consist of deformations of Toda field theory for sl(n). Besides conformal invariance, the theories still enjoy a remnant infinite-dimensional affine symmetry. The case n=3 is used to illustrate this phenomenon, together with further deformations that yield enhanced Kac-Moody symmetry algebras. For generic n we compute N-point correlation functions on the Riemann sphere and show that these can be expressed in terms of sl(n) Toda field theory correlation functions.Comment: 27 pages. Typos corrected. Discussion adde