Nonlocality in time is an important property of systems in which their present state depends on the history of the whole evolution. Combined with the nonlinearity of the process it poses serious difficulties in both analytical and numerical treatment. We investigate a time-fractional porous medium equation that has proved to be important in many applications, notably in hydrology and material sciences. We show that the solution of both free boundary Dirichlet, Neumann, and Robin problems on the half-line satisfies a Volterra integral equation with non-Lipschitz nonlinearity. Based on this result we prove existence, uniqueness, and construct a family of numerical methods that solve these equations outperforming the usual na\"ive finite difference approach. Moreover, we prove the convergence of these methods and illustrate the theory with several numerical examples.
翻译:时间上的不地点性是系统的重要特性,这些系统的现状取决于整个演变的历史。 再加上过程的不线性,在分析和数字处理方面造成了严重的困难。 我们调查了一种在很多应用中,特别是在水文学和物质科学中,证明重要的时差多孔介质方程式。 我们证明,在半线上解决自由边界Drichlet、Neumann和Robin这两个问题的方法既符合沃尔特拉整体方程式,又符合非利普西茨非线性。 基于这个结果,我们证明这些方程式的存在、独一性,并构建了一套数字方法,解决这些方程式的方法超过了通常的“有限度的差别”方法。 此外,我们还证明了这些方法的趋同,并以几个数字例子来说明理论。