Node Removal Vulnerability of the Largest Component of a Network

The connectivity structure of a network can be very sensitive to removal of certain nodes in the network. In this paper, we study the sensitivity of the largest component size to node removals. We prove that minimizing the largest component size is equivalent to solving a matrix one-norm minimization problem whose column vectors are orthogonal and sparse and they form a basis of the null space of the associated graph Laplacian matrix. A greedy node removal algorithm is then proposed based on the matrix one-norm minimization. In comparison with other node centralities such as node degree and betweenness, experimental results on US power grid dataset validate the effectiveness of the proposed approach in terms of reduction of the largest component size with relatively few node removals.Comment: Published in IEEE GlobalSIP 201

PAMAM Dendrimers Undergo pH Responsive Conformational Changes without Swelling

Atomistic molecular dynamics (MD) simulations of a G4-NH2 PAMAM dendrimer were carried out in aqueous solution using explicit water molecules and counterions (with the Dreiding III force field optimized using quantum mechanics). Our simulations predict that the radius of gyration (R_g) of the dendrimer changes little with pH from 21.1 Å at pH 10 (uncharged PAMAM) to 22.1 Å at pH 5 (charged with 126 protons), which agrees quantitatively with recent small angle neutron scattering (SANS) experiments (from 21.4 Å at pH 10 to 21.5 Å at pH 5). Even so we predict a dramatic change in the conformation. The ion pairing in the low pH form leads to a locally compact dense shell with an internal surface area only 37% of the high pH form with a dense core. This transformation from “dense core” at high pH to “dense shell” at low pH could facilitate the encapsulation and release of guest molecules (e.g., drugs) using pH as the trigger, making dendrimers a unique drug delivery vehicle

The Internationalization of Real Estate Research

The contributions of international researchers and institutions to real estate literature for the 1990 to 2006 period are assessed. Both the Asia-Pacific and European regions increase their influence on the top tier of real estate literature. The North American region, while still the dominant source of real estate research, sees its weighted share of publications in the top tier of academic real estate journals decline. Universities from Singapore, Hong Kong, Australia, the United Kingdom, Sweden, and the Netherlands are leaders in their respective regions. Individual researchers domiciled outside of North America are also gaining influence, but few individuals rank high when compared to North American authors. It is anticipated that these trends will continue given the global growth in real estate as an asset class, the importance of real estate investment in countries posting substantial economic growth, and the allocation of resources and human capital within these growing regions to real estate research.

Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts

We report the reaction pathways and barriers for the oxygen reduction reaction (ORR) on platinum, both for gas phase and in solution, based on quantum mechanics calculations (PBE-DFT) on semi-infinite slabs. We find a new mechanism in solution: O_2 → 2O_(ad) (E_(act) = 0.00 eV), O_(ad) + H_2O_(ad) → 2OH_(ad) (E_(act) = 0.50 eV), OH_(ad) + H_(ad) → H_2O_(ad) (E_(act) = 0.24 eV), in which OH_(ad) is formed by the hydration of surface O_(ad). For the gas phase (hydrophilic phase of Nafion), we find that the favored step for activation of the O_2 is H_(ad) + O_(2ad) → HOO_(ad) (E_(act) = 0.30 eV) → HO_(ad) + O_(ad) (E_(act) = 0.12 eV) followed by O_(ad) + H_2O_(ad) → 2OH_(ad) (E_(act) = 0.23 eV), OH_(ad) + H_(ad) → H_2O_(ad) (E_(act) = 0.14 eV). This suggests that to improve the efficiency of ORR catalysts, we should focus on decreasing the barrier for Oad hydration while providing hydrophobic conditions for the OH and H_2O formation steps