Temporalismo e eternismo são teses semânticas. Segundo a primeira, que prevaleceu ao longo da maior parte da história da lógica no Ocidente, existem proposições completas cujo valor de verdade varia com o tempo (proposições temporais), e a investigação das relações entre tempo e verdade é parte integral da lógica. Para a segunda, que emerge gradualmente na obra dos pioneiros da lógica moderna e recebe sua primeira formulação sistemática na obra de Frege, o que o temporalismo percebe como uma proposição com valor de verdade variável deve ser concebido, antes, como uma função proposicional em que pelo menos uma variável livre (usualmente inarticulada na expressão linguística da proposição) toma como argumentos instantes ou intervalos de tempo. As duas teses têm consequências importantes, aqui brevemente resenhadas, para a concepção das relações entre tempo e modalidade e para a semântica das atitudes proposicionais.

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The problem of time‐series discrimination and classification is discussed. We propose a novel clustering algorithm based on a class of quasi U‐statistics and subgroup decomposition tests. The decomposition may be applied to any concave time‐series distance. The resulting test statistics are proven to be asymptotically normal for either i.i.d. or non‐identically distributed groups of time‐series under mild conditions. We illustrate its empirical performance on a simulation study and a real data analysis. The simulation setup includes stationary vs. stationary and stationary vs. non‐stationary cases. The performance of the proposed method is favourably compared with some of the most common clustering measures available.

Beam–plasma instability of inhomogeneous media is an important problem associated with practical applications in space and laboratory plasmas. Previous investigations of the spatially inhomogeneous beam–plasma instability problem assumed one-dimensional velocity and wave number space. This paper extends the available theory to two-dimensional velocity and wave number space. A fully self-consistent set of quasilinear particle and wave kinetic equations is solved for two-dimensional velocity and wave number space in both time and one-dimensional spatial inhomogeneity scale length. The analytical equations include induced and spontaneous emission processes, as well as convective and dispersive effects.

Solar wind electrons near 1 AU feature wide-ranging asymmetries in the superthermal tail distribution. Gaelzer et al. (2008) recently demonstrated that a wide variety of asymmetric distributions results if one considers a pair of counterstreaming electron beams interacting with the core solar wind electrons. However, the nonlinear dynamics was investigated under the simplifying assumption of one dimensionality. In the present paper, this problem is revisited by extending the analysis to two dimensions. The classic bump-on-tail instability involves a single electron beam interacting with the background population. The bidirectional or counterstreaming beams excite Langmuir turbulence initially propagating in opposite directions. It is found that the nonlinear mode coupling leads to the redistribution of wave moments along concentric arcs in wave number space, somewhat similar to the earlier findings by Ziebell et al. (2008) in the case of one beam-plasma instability. However, the present result also shows distinctive features. The similarities and differences in the nonlinear wave dynamics are discussed. It is also found that the initial bidirectional beams undergo plateau formation and broadening in perpendicular velocity space. However, the anisotropy persists in the nonlinear stage, implying that an additional pitch angle scattering by transverse electromagnetic fluctuations is necessary in order to bring the system to a truly isotropic state.

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