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Mehmet Simsek

Publications and source records attributed to Mehmet Simsek.

6 recordsLinked to original sources

Structural Exposure and Subsequent Neighborhood Stability in Temporal Networks: Evidence and Limits of Generalizability

Can temporal path based structural exposure serve as a reliable indicator of subsequent local neighborhood stability across different temporal networks? We investigate this question using the completed observational results from 12 networks spanning face to face nteraction, communication, online activity, and urban mobility. Exposure aggregates source node structural mass along time respecting paths; the outcome is Jaccard neighborhood turnover between later, nonoverlapping windows, with greater turnover indicating lower stability. Six complementary analyses examine the direction and consistency of exposure-turnover associations, proximity to high mass nodes, functional response shapes, distance decay kernels, alternative mass definitions, and temporally ordered changes in source mass. The evidence is heterogeneous rather than uniformly negative or positive. In the primary association analysis, two networks retained positive coefficients after within question false discovery rate adjustment, indicating lower subsequent stability at higher exposure, whereas no negative association survived that adjustment. The pooled proximity coefficient was not distinguishable from zero, and significant mass change associations appeared in opposite directions across networks. More flexible functional, kernel, and mass choices yielded only modest cross validated improvements over simpler comparators; available temporal randomizations also gave model dependent contrasts. The study therefore does not establish structural exposure as a directionally consistent, network independent stability indicator. It also cannot establish a lack of predictive value relative to a no exposure baseline, because that specific comparison and independent cross network validation were unavailable. Incomplete null simulations, unrecorded covariance estimation fallbacks constrain inference.

cs.SI↗

Combined Centrality Measures for an Improved Characterization of Influence Spread in Social Networks

Influence Maximization (IM) aims at finding the most influential users in a social network, i. e., users who maximize the spread of an opinion within a certain propagation model. Previous work investigated the correlation between influence spread and nodal centrality measures to bypass more expensive IM simulations. The results were promising but incomplete, since these studies investigated the performance (i. e., the ability to identify influential users) of centrality measures only in restricted settings, e. g., in undirected/unweighted networks and/or within a propagation model less common for IM. In this paper, we first show that good results within the Susceptible- Infected-Removed (SIR) propagation model for unweighted and undirected networks do not necessarily transfer to directed or weighted networks under the popular Independent Cascade (IC) propagation model. Then, we identify a set of centrality measures with good performance for weighted and directed networks within the IC model. Our main contribution is a new way to combine the centrality measures in a closed formula to yield even better results. Additionally, we also extend gravitational centrality (GC) with the proposed combined centrality measures. Our experiments on 50 real-world data sets show that our proposed centrality measures outperform well-known centrality measures and the state-of-the art GC measure significantly. social networks, influence maximization, centrality measures, IC propagation model, influential spreaders

cs.SI↗

The Klein-Gordon Equation of Generalized Hulthen Potential in Complex Quantum Mechanics

We have investigated the reality of exact bound states of complex and/or PT-symmetric non-Hermitian exponential-type generalized Hulthen potential. The Klein-Gordon equation has been solved by using the Nikiforov-Uvarov method which is based on solving the second-order linear differential equations by reduction to a generalized equation of hypergeometric type. In many cases of interest, negative and positive energy states have been discussed for different types of complex potentials.

quant-ph↗

Bound States of Non-Hermitian Quantum Field Theories

The spectrum of the Hermitian Hamiltonian ${1\over2}p^2+{1\over2}m^2x^2+gx^4$ ($g>0$), which describes the quantum anharmonic oscillator, is real and positive. The non-Hermitian quantum-mechanical Hamiltonian $H={1\over2}p^2+{1 \over2}m^2x^2-gx^4$, where the coupling constant $g$ is real and positive, is ${\cal PT}$-symmetric. As a consequence, the spectrum of $H$ is known to be real and positive as well. Here, it is shown that there is a significant difference between these two theories: When $g$ is sufficiently small, the latter Hamiltonian exhibits a two-particle bound state while the former does not. The bound state persists in the corresponding non-Hermitian ${\cal PT}$-symmetric $-gϕ^4$ quantum field theory for all dimensions $0\leq D<3$ but is not present in the conventional Hermitian $gϕ^4$ field theory.

hep-th↗

Quantum Complex Henon-Heiles Potentials

Quantum-mechanical PT-symmetric theories associated with complex cubic potentials such as V=x^2+y^2+igxy^2 and V=x^2+y^2+z^2+igxyz, where g is a real parameter, are investigated. These theories appear to possess real, positive spectra. Low-lying energy levels are calculated to very high order in perturbation theory. The large-order behavior of the perturbation coefficients is determined using multidimensional WKB tunneling techniques. This approach is also applied to the complex Henon-Heiles potential V=x^2+y^2+ig(xy^2-x^3/3).

quant-ph↗