We give a multimensional version of the p-adic heat equation, and show that its fundamental solution is the transition density of a Markov process. 1. Introduction. In recent years p adic analysis has received a lot of attention due to its applications in mathematical physics, see e.g. [1], [2], [4], [5], [16], [17], [18], [22], [28] and references therein. One motivation comes from statistical physics, in particular in connection with models describing relaxation in glasses, macromolecules, and proteins. It has been proposed that the non exponential nature of those relaxations is a consequence of a hierarchical structure of the state space which can in turn be put in connection with p adic structures ([4], [5], [22]). In [4] was demostrated that the p-adic analysis is a natural basis for the construction of a wide variety of models of ultrametric di¤usion constrained by hierarchical energy landscapes. To each of these models is associated a stochastic equation (the master equation). In several cases this equation is a p-adic parabolic equation of type: > : @u(x;t) @t + a(Au)(x; t) = f(x; t); x 2 Q n p ; t 2 (0; T ]; u(x; 0) = '(x); (1.1) where a is a positive constant, A is pseudo-di¤erential operator, and Qp is the
eld of p-adic numbers. The simplest case occurs when n = 1 and A is the Vladimirov operator: (D ') (x) = F 1 !x j j p Fx! '(x) ; > 0; where F is the Fourier transform. The fundamental solution of (1.1) is density transition of a timeand space-homogeneous Markov process, that is consider the p adic counterpart of the Brownian motion (see [18], [28]). It is relevant to mention that in the case n = 1, the fundamental solution of (1.1) when A = D (also called the p adic heat kernel) has been studied extensively, see e.g. [6], [11], [12], [14], [18], [28]. A natural problem is to study the initial value problem (1.1) in the n-dimensional case. Recently, the second author considered Cauchys problem (1.1) when (A') (x) = F 1 !x jf ( )j p Fx! '(x) ; > 0; 2000 Mathematics Subject Classi
cation. Primary 35R60, 60J25; Secondary 47S10, 35S99. Key words and phrases. Parabolic equations, Markov processes, p-adic numbers, ultrametric di¤usion. 1 2 J. J. RODRIGUEZ-VEGA AND W. A. ZUNIGA-GALINDO here f ( ) is an elliptic homogeneous polynomial in n variables, and the datum ' is a locally constant and integrable function. Under these hypotheses it was established the existence of a unique solution to Cauchys problem (1.1). In addition, the fundamental solution is a transition density of a Markov process with space state Qp (see [29]). In this paper we study Cauchys problem (1.1) when A is the Taibleson pseudodi¤erential operator which is de
ned as follows: D T' (x) = F 1 !x max 1 i n j ijp Fx! '(x) ! ; > 0: (1.2) Recently Albeverio, Khrennikov, and Shelkovich studied D T in the context of the Lizorkin spaces [3]. We prove existence and uniqueness of the Cauchy problem (1.1-1.2) in spaces of increasing functions introduced by Kochubei in [19], see Theorem 1. We also associate a Markov processes to equation the fundamental solution (see Theorem 2). These results constitute an extension of the corresponding results in [18], [28]. Let us explain the connection between the results of this paper and those of [29]. There are in
nitely many homogeneous polynomial functions satisfying jf ( )jp = max 1 i n j ijp d ; for any 2 Qp ; here d denotes degree of f (c.f. Lemmas 14-15). Hence the pseudo-di¤erential operators considered here are a subclass of the ones considered in [29]. However, the function spaces for the solutions and initial data are completely di¤erent. In this paper the initial datum and the solution to Cauchy problem (1.1-1.2) are not necessarily bounded, neither integrable, but in [29] are. Finally, our results can be extended to operators of the form (A') (x) = a0(x; t)(D T')(x) + n X k=1 ak(x; t)(D k T ')(x) + b(x; t)'(x); (1.3) > 1, 0 < 1 < : : : < n < ; where the ak(x; t) ,and b(x; t) are bounded continuous functions, using the techniques presented in [18]-[20]. These results will appear later elsewhere. 2. Preliminary Results As general reference for p-adic analysis we refer the reader to [25] and [28]. The
eld of p-adic numbers Qp is de
ned as the completion of the
eld of rational numbers Q with respect to the non-Archimedean p-adic norm j jp which is de
ned as follows: j0jp = 0; if x 2 Q , x = p ab with a, b integers coprime to p, then jxjp = p . The integer = (x) is called the p-adic order of x, and it will be denoted as ord (x). We use the same symbol, j jp, for the p-adic norm on Qp. We extend the p-adic norm to Qp as follows: kxkp := max 1 i n jxijp , for x = (x1; : : : ; xn) 2 Q n p . Note that kxkp = p min1 i nford(xi)g. p-ADIC PARABOLIC EQUATIONS AND ULTRAMETRIC DIFFUSION 3 Any p-adic number x 6= 0 has a unique expansion of the form