Diffusion in sparse networks: linear to semi-linear crossover

We consider random networks whose dynamics is described by a rate equation, with transition rates $w_{nm}$ that form a symmetric matrix. The long time evolution of the system is characterized by a diffusion coefficient $D$. In one dimension it is well known that $D$ can display an abrupt percolation-like transition from diffusion ($D>0$) to sub-diffusion (D=0). A question arises whether such a transition happens in higher dimensions. Numerically $D$ can be evaluated using a resistor network calculation, or optionally it can be deduced from the spectral properties of the system. Contrary to a recent expectation that is based on a renormalization-group analysis, we deduce that $D$ is finite; suggest an "effective-range-hopping" procedure to evaluate it; and contrast the results with the linear estimate. The same approach is useful for the analysis of networks that are described by quasi-one-dimensional sparse banded matrices.Comment: 13 pages, 4 figures, proofed as publishe

Consumption and Risk Sharing Over the Life Cycle

A striking feature of U.S. data on income and consumption is that inequality increases with age. This paper asks if individual-specific earnings risk can provide a coherent explanation. We find that it can. We construct an overlapping generations general equilibrium model in which households face uninsurable earnings shocks over the course of their lifetimes. Earnings inequality is exogenous and is calibrated to match data from the U.S. Panel Study on Income Dynamics. Consumption inequality is endogenous and matches well data from the U.S. Consumer Expenditure Survey. The total risk households face is decomposed into that realized before entering the labor market and that realized throughout the working years. In welfare terms, the latter is found to be more important than the former

Asset Pricing With Idiosyncratic Risk and Overlapping Generations

What is the effect of non-tradeable idiosyncratic risk on asset-market risk premiums? Constantinides and Duffie [Constantinides, G.M., Duffie, D., 1996. Asset pricing with heterogeneous consumers. Journal of Political Economy 104, 219–240] and Mankiw [Mankiw, N.G., 1986. The equity premium and the concentration of aggregate shocks. Journal of Financial Economics 17, 211–219] have shown that risk premiums will increase if the idiosyncratic shocks become more volatile during economic contractions. We add two important ingredients to this relationship: (i) the life cycle, and (ii) capital accumulation. We show that in a realistically-calibrated life-cycle economy with production these ingredients mitigate the ability of idiosyncratic risk to account for the observed Sharpe ratio on US equity. While the Constantinides–Duffie model can account for the US value of 41% with a risk-aversion coefficient of 8, our model generates a Sharpe ratio of 33%, which is roughly half-way to the complete-markets value of 25%. Almost all of this reduction is due to capital accumulation. Life-cycle effects are important in our model—we demonstrate that idiosyncratic risk matters for asset pricing because it inhibits the intergenerational sharing of aggregate risk—but their net effect on the Sharpe ratio is small

Shocks and Universal Statistics in (1+1)-Dimensional Relativistic Turbulence

We propose that statistical averages in relativistic turbulence exhibit universal properties. We consider analytically the velocity and temperature differences structure functions in the (1+1)-dimensional relativistic turbulence in which shock waves provide the main contribution to the structure functions in the inertial range. We study shock scattering, demonstrate the stability of the shock waves, and calculate the anomalous exponents. We comment on the possibility of finite time blowup singularities.Comment: 37 pages, 7 figure

Consumption and Risk Sharing Over the Life Cycle

A striking feature of U.S. data on income and consumption is that inequality increases with age. Using both panel data and an equilibrium life cycle model, we argue that this is informative for understanding the importance and the characteristics of idiosyncratic labor market risk. We find that uncertainty distributed throughout the working years accounts for 40 percent of life time uncertainty, with the remainder being realized prior to entering the labor market. We estimate that the shocks received over the life cycle contain a highly persistent component, with an autocorrelation coefficient between 0.98 and unity. The joint behavior of earnings and consumption inequality, interpreted using our model, adds to the body of evidence suggesting that labor market risks are imperfectly pooled and that a precautionary motive is an important aspect of U.S. savings behavior. The restrictions imposed by general equilibrium theory play an important role in arriving at each of these conclusions.

The Pure Spinor Formulation of Superstrings

In this lectures we outline the construction of pure spinor superstrings. We consider both the open and closed pure spinor superstrings in critical and noncritical dimensions and on flat and curved target spaces with RR flux. We exhibit the integrability properties of pure spinor superstrings on curved backgrounds with RR fluxes.Comment: These lectures have been given in the RTN Winter School on Strings, Supergravity and Gauge Theories, CERN (2008). 32 pages, a typo correcte

Analysis of the distribution and structure of integrated banana streak virus DNA in a range of Musa cultivars

The cDNA encoding the glycoprotein α (GPα) subunit of tilapia (Oreochromis mossambicus) was partially cloned using RACE-polymerase chain reaction (PCR) technique. The amplified cDNA was found to be 583 bases long, and to consist of a portion of the signal peptide, the full sequence encoding the mature peptide (94 amino acids) and the 3′ untranslated region. Northern blot analysis revealed a single band of approximately 600 bp. Alignment of the deduced amino acids of the mature protein showed that the tilapia GPα subunit shares more than 80% identity with that of other perciform fish (i.e. striped bass, sea bream and yellowfin porgy) and less than 70% with that of more taxonomically remote fish and other vertebrates. Exposure of dispersed tilapia pituitary cells to salmon gonadotropin-releasing hormone (sGnRH) elevated GPα mRNA levels via both PKC and cAMP-protein kinase A (PKA) pathways. The transcript levels were also regulated by pituitary adenylate cyclase activating polypeptide (PACAP) and neuropeptide Y (NPY), both acting through PKC and PKA pathways. Moreover, a combined treatment of PACAP or NPY with GnRH seems to have an additive effect on the GPα subunit gene transcription. These results suggest that in tilapia the expression of GPα subunit is regulated by GnRH mainly via PKC and PKA pathways. Furthermore, PACAP and NPY can elevate the GnRH-stimulated GPα subunit transcription and can directly affect the subunit mRNA levels, via the same transduction pathways

Close-packed floating clusters: granular hydrodynamics beyond the freezing point?

Monodisperse granular flows often develop regions with hexagonal close packing of particles. We investigate this effect in a system of inelastic hard spheres driven from below by a "thermal" plate. Molecular dynamics simulations show, in a wide range of parameters, a close-packed cluster supported by a low-density region. Surprisingly, the steady-state density profile, including the close-packed cluster part, is well described by a variant of Navier-Stokes granular hydrodynamics (NSGH). We suggest a simple explanation for the success of NSGH beyond the freezing point.Comment: 4 pages, 5 figures. To appear in Phys. Rev. Let

Dynamical model of the dielectric screening of conjugated polymers

A dynamical model of the dielectric screening of conjugated polymers is introduced and solved using the density matrix renormalization group method. The model consists of a line of quantized dipoles interacting with a polymer chain. The polymer is modelled by the Pariser-Parr-Pople (P-P-P) model. It is found that: (1) Compared to isolated, unscreened single chains, the screened 1Bu- exciton binding energy is typically reduced by ca. 1 eV to just over 1 eV; (2) Covalent (magnon and bi-magnon) states are very weakly screened compared to ionic (exciton) states; (3) Screening of the 1Bu- exciton is closer to the dispersion than solvation limit.Comment: 12 pages, 2 figure