We treat the time evolution of states on a finite directed graph, with singular diffusion on the edges of the graph and glueing conditions at the vertices. The operator driving the evolution is obtained by the method of quadratic forms on a suitable Hilbert space. Using the Beurling-Deny criteria we describe glueing conditions leading to positive and to submarkovian semigroups, respectively.
IntroductionThe intentions of this paper are twofold. The first aim is to present a treatment of one-dimensional "singular" diffusion in the framework of Dirichlet forms. The second is to present suitable boundary or glueing conditions on graphs (quantum graphs) leading to positive or submarkovian C 0 -semigroups.Concerning the first topic we assume that µ is a finite Borel measure on a bounded interval [a, b]. We assume that particles move in [a, b] according to "Brownian motion" but are only allowed to be located in the support of µ, and in fact are slowed down or accelerated by the "speed measure" µ. (Incidentally, the support of µ is allowed to have gaps, what sometimes is referred to by "gap diffusion".) More generally, instead of starting with Brownian motion, one also can include a drift in the diffusion. This leads to including a scale function. The treatment of the corresponding process has a longer history (cf. [1,3-5,12-14,19,20,22]), but there appears to be no treatment of the arising evolution in the context of Dirichlet forms.Concerning the second topic, we assume that finitely many intervals, with diffusion as described above, are arranged in a graph, and we treat the question how boundary conditions (glueing conditions) at the vertices can be posed in a way to describe diffusing particles. These topics have also been treated in a recent paper by Kostrykin et al. [16]. Since we pose the glueing conditions in a different form (following [17]) it does not seem evident to us to establish the connection between the conditions given in [16] and our conditions.There are many motivations from applications for this work. Since we were not primarily motivated by specific applications we refer to [17,18,23] for some of these Mathematics Subject Classification (2000): 47D06, 60J60, 47E05, 35Q99, 05C99