arXiv subjects

Thomas Michelitsch

Publications and source records attributed to Thomas Michelitsch.

16 recordsLinked to original sources

arXiv2023

Compartment model with retarded transition rates

Our study is devoted to a four-compartment epidemic model of a constant population of independent random walkers. Each walker is in one of four compartments (S-susceptible, C-infected but not infectious (period of incubation), I-infected and infectious, R-recovered and immune) characterizing the states of health. The walkers navigate independently on a periodic 2D lattice. Infections occur by collisions of susceptible and infectious walkers. Once infected, a walker undergoes the delayed cyclic transition pathway S $\to$ C $\to$ I $\to$ R $\to$ S. The random delay times between the transitions (sojourn times in the compartments) are drawn from independent probability density functions (PDFs). We analyze the existence of the endemic equilibrium and stability of the globally healthy state and derive a condition for the spread of the epidemics which we connect with the basic reproduction number $R_0>1$. We give quantitative numerical evidence that a simple approach based on random walkers offers an appropriate microscopic picture of the dynamics for this class of epidemics.

q-bio.PE↗

arXiv2018

Development of aging changes: self-accelerating and inhomogeneous

Aging changes including age spots and atherosclerotic plaques develop in an inhomogeneous and accelerated manner. For understanding this phenomenon, some aging changes are analyzed by Misrepair mechanism, a mechanism proposed in Misrepair-accumulation theory. I. Misrepair is a strategy of repair for survival of an organism in situations of severe injuries; however a Misrepair alters the structure of a tissue, a cell or a molecule, which are the sub-structures of an organism. II. Misrepair of a sub-structure also alters the spatial relationship of this sub-structure with its neighbor sub-structures. Thus a Misrepair leads to increased damage-sensitivity and reduced repair-efficiency of these sub-structures. As a result, Misrepairs have a tendency to occur to the sub-structure and its neighbor sub-structures where an old Misrepair has taken place. In return, new Misrepairs will increase again the damage-sensitivity of these sub-structures and the surrounding sub-structures. By such a vicious circle, the frequency of Misrepairs to these sub-structures is increased and the range of affected sub-structures is enlarged after each time of Misrepair. Thus, accumulation of Misrepairs is focalized and self-accelerating. III. Focalized accumulation of Misrepairs leads to formation and growing of a "spot" or "plaque" in a tissue. Growing of a spot is self-accelerating, and old spots grow faster than new ones. New spots tend to develop close to old ones, resulting in an inhomogeneous distribution of spots. In conclusion, the inhomogeneous development of aging changes is a result of self-accelerated and focalized accumulation of Misrepairs; and the process of aging is self-accelerating.

q-bio.OT↗

arXiv2017

Traditional aging theories: which ones are useful?

Many theories have been proposed to answer two questions on aging: "Why do we age?" and "How do we age?" Among them, evolutionary theories are proposed to interpret the evolutionary advantage of aging, and "saving resources for group benefit" is thought to be the purpose of aging. However for saving resources, a more economic strategy should be to make a rapid death to the individuals who are over the reproduction age rather than to make them aging. Biological theories are proposed to identify the causes and the biological processes of aging. However, some theories including cell senescence/telomere theory, gene-controlling theory, and developmental theory, have unfortunately ignored the influence of damage on aging. Free-radical theory suggests that free radicals by causing intrinsic damage are the main cause of aging. However, even if intracellular free radicals cause injuries, they could be only associated with some but not all of the aging changes. Damage (fault)-accumulation theory predicts that faults as intrinsic damage can accumulate and lead to aging. However, in fact an unrepaired fault could not possibly remain in a living organism, since it can destroy the integrity of tissue structure and cause rapid failure of the organism. These traditional theories are all incomplete on interpreting aging phenomena. Nevertheless, developmental theory and damage (fault)-accumulation theory are more useful, because they have recognized the importance of damage and development process in aging. Some physical theories are useful, because they point out the common characteristics of aging changes, including loss of complexity, consequence of increase of entropy, and failure of information-transmission. An advanced theory, which can include all of these useful ideas in traditional theories, is needed.

q-bio.OT↗

arXiv2017

Aging as a process of accumulation of Misrepairs

We recently introduced Misrepair-accumulation theory as an interpretation on aging mechanism. For better understanding this theory, we discuss here in more details the new concept of Misrepair and the concept of accumulation of Misrepairs. I. Aging takes place uniquely on the systems that have a well-defined structure: an organization of its sub-structures. Aging of a non-living system is a result of accumulation of injuries (damage) of its structure. II. A generalized concept of Misrepair is important for reaching to a unified understanding of aging changes. In the situation of a severe injury, incorrect repair is a repair essential for maintaining structural integrity for increasing the surviving chance of an organism. With the same mechanism as that in "Misrepair of DNA" , the term of "Misrepair" can be used for describing all kinds of incorrect repairs on different types of living structures, including molecules, cells and tissues. A new concept of Misrepair is therefore proposed and defined as incorrect reconstruction of an injured living structure. Misrepair mechanism is beneficial for the survival of an organism, and it is essential for the survival of a species. III. Alteration of structure made by Misrepair is irreversible; therefore Misrepairs accumulate and disorganize gradually a structure, which appears as aging of it. Accumulation of Misrepairs takes place on molecular, cellular and tissue levels respectively: on tissue level, it appears as disorganization of cells and extracellular matrixes (ECMs); on cellular level, it appears as deformation of cytoskeleton and change of cell shape; and on DNA, it appears as accumulation of DNA mutations and alteration of DNA sequence. An essential change in aging of an organism is an irreversible change of the spatial relationship between cells/ECMs in a tissue. In conclusion, aging of an organism is a result of accumulation of Misrepairs on tissue level.

q-bio.TO↗

arXiv2016

Fractional random walk lattice dynamics

We analyze time-discrete and continuous `fractional' random walks on undirected regular networks with special focus on cubic periodic lattices in $n=1,2,3,..$ dimensions. The fractional random walk dynamics is governed by a master equation involving {\it fractional} powers of Laplacian matrices $L^{\fracα{2}}$}where $α=2$ recovers the normal walk. First we demonstrate that the interval $0<α\leq 2$ is admissible for the fractional random walk. We derive analytical expressions for fractional transition matrix and closely related the average return probabilities. We further obtain the fundamental matrix $Z^{(α)}$, and the mean relaxation time (Kemeny constant) for the fractional random walk. The representation for the fundamental matrix $Z^{(α)}$ relates fractional random walks with normal random walks. We show that the fractional transition matrix elements exhibit for large cubic $n$-dimensional lattices a power law decay of an $n$-dimensional infinite space Riesz fractional derivative type indicating emergence of Lévy flights. As a further footprint of Lévy flights in the $n$-dimensional space, the fractional transition matrix and fractional return probabilities are dominated for large times $t$ by slowly relaxing long-wave modes leading to a characteristic $t^{-\frac{n}α}$-decay. It can be concluded that, due to long range moves of fractional random walk, a small world property is emerging increasing the efficiency to explore the lattice when instead of a normal random walk a fractional random walk is chosen.

cond-mat.stat-mech↗

arXiv2016

Fractional Lattice Dynamics: Nonlocal constitutive behavior generated by power law matrix functions and their fractional continuum limit kernels

We introduce positive elastic potentials in the harmonic approximation leading by Hamilton's variational principle to fractional Laplacian matrices having the forms of power law matrix functions of the simple local Bornvon Karman Laplacian. The fractional Laplacian matrices are well defined on periodic and infinite lattices in $n=1,2,3,..$ dimensions. The present approach generalizes the central symmetric second differenceoperator (Born von Karman Laplacian) to its fractional central symmetric counterpart (Fractional Laplacian matrix).For non-integer powers of the Born von Karman Laplacian, the fractional Laplacian matrix is nondiagonal with nonzero matrix elements everywhere, corresponding to nonlocal behavior: For large lattices the matrix elements far from the diagonal expose power law asymptotics leading to continuum limit kernels of Riesz fractional derivative type. We present explicit results for the fractional Laplacian matrix in 1D for finite periodic and infinite linear chains and their Riesz fractional derivative continuum limit kernels.The approach recovers for $α=2$ the well known classical Born von Karman linear chain (1D lattice) with local next neighbor springsleading in the well known continuum limit of classic local standard elasticity, and for other integer powers to gradient elasticity.We also present a generalization of the fractional Laplacian matrix to n-dimensional cubic periodic (nD tori) and infinite lattices. For the infinite nD lattice we deducea convenient integral representation.We demonstrate that our fractional lattice approach is a powerful tool to generate physically admissible nonlocal lattice material models and their continuum representations.

math-ph↗

arXiv2015

Lattice fractional Laplacian and its continuum limit kernel on the finite cyclic chain

The aim of this paper is to deduce a discrete version of the fractional Laplacian in matrix form defined on the 1D periodic (cyclically closed) linear chain of finite length.We obtain explicit expressions for this fractional Laplacianmatrix and deduce also its periodic continuum limit kernel. The continuum limit kernel gives an exact expression for the fractional Laplacian (Riesz fractional derivative) on the finite periodic string.In this approach we introduce two material parameters, the particle mass $μ$ anda frequency $Ω\_α$. The requirement of finiteness of the the total mass and total elastic energy in the continuum limit (lattice constant $h\rightarrow 0$) leads to scaling relations for the two parameters, namely$μ\sim h$ and $Ω\_α^2\sim h^{-α}$.The present approach can be generalized to define lattice fractional calculus on periodic lattices in full analogy to the usual `continuous' fractional calculus.

math-ph↗

arXiv2015

Calculation of the Electroelastic Green's Function of the Hexagonal Infinite Medium

The electroelastic 4 $\times$ 4 Green's function of a piezoelectric hexagonal (transversely isotropic) infinitely extended medium is calculated explicitly in closed compact form (eqs. (73) ff. and (88) ff., respectively) by using residue calculation. The results can also be derived from Fredholm's method [2]. In the case of vanishing piezoelectric coupling the derived Green's function coincides with two well known results: Kr{ö}ner 's expressions for the elastic Green's function tensor [4] is reproduced and the electric part then coincides with the electric potential (solution of Poisson equation) which is caused by a unit point charge. The obtained electroelastic Green's function is useful for the calculation of the electroelastic Eshelby tensor [16].

math-ph↗

arXiv2015

A regularized representation of the fractional Laplacian in n dimensions and its relation to Weierstrass-Mandelbrot type fractal functions

We demonstrate that the fractional Laplacian (FL) is the principal characteristic operator of harmonic systems with {\it self-similar} interparticle interactions. We show that the FL represents the "{\it fractional continuum limit}" of a discrete "self-similar Laplacian" which is obtained by Hamilton's variational principle from a discrete spring model. We deduce from generalized self-similar elastic potentials regular representations for the FL which involve convolutions of symmetric finite difference operators of even orders extending the standard representation of the FL. Further we deduce a regularized representation for the FL $-(-Δ)^{\fracα{2}}$ holding for $α\in \R \geq 0$. We give an explicit proof that the regularized representation of the FL gives for integer powers $\fracα{2} \in \N\_0$ a distributional representation of the standard Laplacian operator $Δ$ including the trivial unity operator for $α\rightarrow 0$. We demonstrate that self-similar {\it harmonic} systems are {\it all} governed in a distributional sense by this {\it regularized representation of the FL} which therefore can be conceived as characteristic footprint of self-similarity.

math-ph↗

arXiv2014

Fractional Laplacian matrix on the finite periodic linear chain and its periodic Riesz fractional derivative continuum limit

The 1D discrete fractional Laplacian operator on a cyclically closed (periodic) linear chain with finitenumber $N$ of identical particles is introduced. We suggest a "fractional elastic harmonic potential", and obtain the $N$-periodic fractionalLaplacian operator in the form of a power law matrix function for the finite chain ($N$ arbitrary not necessarily large) in explicit form.In the limiting case $N\rightarrow \infty$ this fractional Laplacian matrix recovers the fractional Laplacian matrix ofthe infinite chain.The lattice model contains two free material constants, the particle mass $μ$ and a frequency$Ω\_α$.The "periodic string continuum limit" of the fractional lattice model is analyzed where lattice constant $h\rightarrow 0$and length $L=Nh$ of the chain ("string") is kept finite: Assuming finiteness of the total mass and totalelastic energy of the chain in the continuum limit leads to asymptotic scaling behavior for $h\rightarrow 0$ of thetwo material constants,namely $μ\sim h$ and $Ω\_α^2 \sim h^{-α}$. In this way we obtain the $L$-periodic fractional Laplacian (Riesz fractional derivative) kernel in explicit form.This $L$-periodic fractional Laplacian kernel recovers for $L\rightarrow\infty$the well known 1D infinite space fractional Laplacian (Riesz fractional derivative) kernel. When the scaling exponentof the Laplacian takesintegers, the fractional Laplacian kernel recovers, respectively, $L$-periodic and infinite space (localized) distributionalrepresentations of integer-order Laplacians.The results of this paper appear to beuseful for the analysis of fractional finite domain problems for instance in anomalous diffusion (Lévy flights), fractional Quantum Mechanics,and the development of fractional discrete calculus on finite lattices.

math-ph↗

arXiv2013

Nonlocal constitutive laws generated by matrix functions: Lattice Dynamics Models and their Continuum Limits

We analyze one-dimensional discrete and quasi-continuous linear chains of $N>>1$ equidistant and identical mass points with periodic boundary conditions and generalized nonlocal interparticle interactions in the harmonic approximation. We introduce elastic potentials which define by Hamilton's principle discrete "Laplacian operators" ("Laplacian matrices") which are operator functions ($N\times N$-matrix functions) of the Laplacian of the Born-von-Karman linear chain with next neighbor interactions. The non-locality of the constitutive law of the present model is a natural consequence of the {\it non-diagonality} of these Laplacian matrix functions in the $N$ dimensional vector space of particle displacement fields where the periodic boundary conditions (cyclic boundary conditions) and as a consequence the (Bloch-) eigenvectors of the linear chain are maintained. In the quasi-continuum limit (long-wave limit) the Laplacian matrices yield "Laplacian convolution kernels" (and the related elastic modulus kernels) of the non-local constitutive law. The elastic stability is guaranteed by the positiveness of the elastic potentials. We establish criteria for "weak" and "strong" nonlocality of the constitutive behavior which can be controlled by scaling behavior of material constants in the continuum limit when the interparticle spacing $h\rightarrow 0$. The approach provides a general method to generate physically admissible (elastically stable) {\it non-local constitutive laws} by means of "simple" Laplacian matrix functions. The model can be generalized to model non-locality in $n=2,3,..$ dimensions of the physical space.

math-ph↗

arXiv2012

An approach to anomalous diffusion in the n-dimensional space generated by a self-similar Laplacian

We analyze a quasi-continuous linear chain with self-similar distribution of harmonic interparticle springs as recently introduced for one dimension (Michelitsch et al., Phys. Rev. E 80, 011135 (2009)). We define a continuum limit for one dimension and generalize it to $n=1,2,3,..$ dimensions of the physical space. Application of Hamilton's (variational) principle defines then a self-similar and as consequence non-local Laplacian operator for the $n$-dimensional space where we proof its ellipticity and its accordance (up to a strictly positive prefactor) with the fractional Laplacian $-(-Δ)^\fracα{2}$. By employing this Laplacian we establish a Fokker Planck diffusion equation: We show that this Laplacian generates spatially isotropic Lévi stable distributions which correspond to Lévi flights in $n$-dimensions. In the limit of large scaled times $\sim t/r^α >>1$ the obtained distributions exhibit an algebraic decay $\sim t^{-\frac{n}α} \rightarrow 0$ independent from the initial distribution and spacepoint. This universal scaling depends only on the ratio $n/α$ of the dimension $n$ of the physical space and the Lévi parameter $α$.

physics.class-ph↗

arXiv2011

Sur une généralisation de l'opérateur fractionnaire

The goal of this communication is to propose a generalized notion of the "traditional derivative". This generalization includes the fractional derivatives such as the Riemann-Liouville, Gruenwald-Letnikov, Weyl, Riesz, Caputo, Marchaud derivatives and other variants as special cases. The approach is useful to describe mechanical problems in material systems with microstructures without characteristic scale such as self-similar and fractal materials.

physics.class-ph↗

arXiv2011

A self-similar field theory for 1D linear elastic continua and self-similar diffusion problem

This paper is devoted to the analysis of some fundamental problems of linear elasticity in 1D continua with self-similar interparticle interactions. We introduce a self-similar continuous field approach where the self-similarity is reflected by equations of motion which are spatially non-local convolutions with power-function kernels (fractional integrals). We obtain closed-form expressions for the static displacement Green's function due to a unit $δ$-force. In the dynamic framework we derive the solution of the {\it Cauchy problem} and the retarded Green's function. We deduce the distribution of a self-similar variant of diffusion problem with Lévi-stable distributions as solutions with infinite mean fluctuations describing the statistics Lévi-flights. We deduce a hierarchy of solutions for the self-similar Poisson's equation which we call "self-similar potentials". These non-local singular potentials are in a sense self-similar analogues to the 1D-Dirac's $δ$-function. The approach can be the starting point to tackle a variety of scale invariant interdisciplinary problems.

math-ph↗

arXiv2011

Aging as a consequence of misrepair -- a novel theory of aging

It is now increasingly realized that the underlying mechanism which governs aging (ageing) is a complex interplay of genetic regulation and damage-accumulation. "Aging as a result of accumulation of 'faults' on cellular and molecular levels", has been proposed in the damage (fault)-accumulation theory. However, this theory fails to explain some aging phenotypes such as fibrosis and premature aging, since terms such as 'damage' and 'fault' are not specified. Therefore we introduce some crucial modifications of this theory and arrive at a novel theory: aging of the body is the result of accumulation of Misrepair of tissue. It emphasizes: a) it is Misrepair, not the original damage, that accumulates and leads to aging; and b) aging can occur at different levels, however aging of the body takes place necessarily on the tissue level, but not requiring the aging of cells/molecules. The Misrepair-accumulation theory introduced in the present paper unifies the understanding of the roles of environmental damage, repair, gene regulation, and multicellular structure in the aging process. This theory gives explanations for the aging phenotypes, premature aging, the difference of longevity in different species, and it is consistent with the physical view on complex systems.

q-bio.TO↗

arXiv2009

Analysis of the Vibrational Mode Spectrum of a Linear Chain with Spatially Exponential Properties

We deduce the dynamic frequency-domain-lattice Green's function of a linear chain with properties (masses and next-neighbor spring constants) of exponential spatial dependence. We analyze the system as discrete chain as well as the continuous limiting case which represents an elastic 1D exponentially graded material. The discrete model yields closed form expressions for the $N \times N$ Green's function for an arbitrary number $N=2,..,\infty$ of particles of the chain. Utilizing this Green's function yields an explicit expression for the vibrational mode density. Despite of its simplicity the model reflects some characteristics of the dynamics of a 1D exponentially graded elastic material. As a special case the well-known expressions for the Green's function and oscillator density of the homogeneous linear chain are contained in the model. The width of the frequency band is determined by the grading parameter which characterizes the exponential spatial dependence of the properties. In the limiting case of large grading parameter, the frequency band is localized around a single finite frequency where the band width tends to zero inversely with the grading parameter. In the continuum limit the discrete Green's function recovers the Green's function of the continuous equation of motion which takes in the time domain the form of a Klein-Gordon equation.

math.SP↗