Regularity Properties and Pathologies of Position-Space Renormalization-Group Transformations

We reconsider the conceptual foundations of the renormalization-group (RG) formalism, and prove some rigorous theorems on the regularity properties and possible pathologies of the RG map. Regarding regularity, we show that the RG map, defined on a suitable space of interactions (= formal Hamiltonians), is always single-valued and Lipschitz continuous on its domain of definition. This rules out a recently proposed scenario for the RG description of first-order phase transitions. On the pathological side, we make rigorous some arguments of Griffiths, Pearce and Israel, and prove in several cases that the renormalized measure is not a Gibbs measure for any reasonable interaction. This means that the RG map is ill-defined, and that the conventional RG description of first-order phase transitions is not universally valid. For decimation or Kadanoff transformations applied to the Ising model in dimension $d \ge 3$, these pathologies occur in a full neighborhood $\{ \beta > \beta_0 ,\, |h| < \epsilon(\beta) \}$ of the low-temperature part of the first-order phase-transition surface. For block-averaging transformations applied to the Ising model in dimension $d \ge 2$, the pathologies occur at low temperatures for arbitrary magnetic-field strength. Pathologies may also occur in the critical region for Ising models in dimension $d \ge 4$. We discuss in detail the distinction between Gibbsian and non-Gibbsian measures, and give a rather complete catalogue of the known examples. Finally, we discuss the heuristic and numerical evidence on RG pathologies in the light of our rigorous theorems.Comment: 273 pages including 14 figures, Postscript, See also ftp.scri.fsu.edu:hep-lat/papers/9210/9210032.ps.

Existence theorems concerning simple integrals of the calculus of variations for discontinuous solutions

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46210/1/205_2004_Article_BF00276912.pd

Existence theorems for multiple integrals of the calculus of variations for discontinuous solutions

The authors prove existence theorems for the minimum of multiple integrals of the calculus of variations with constraints on the derivatives in classes of BV possibly discontinuous solutions. To this effect the integrals are written in the form proposed by Serrin. Usual convexity conditions are requested, but no growth condition. Preliminary closure and semicontinuity theorems are proved which are analogous to those previously proved by Cesari in Sobolev classes. Compactness in L 1 of classes of BV functions with equibounded total variations is derived from Cafiero-Fleming theorems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47939/1/10231_2005_Article_BF01766143.pd

Satistical problems of point processes

Wykłady niniejsze wygłoszone były we wrześniu 1976 r. w Międzynarodowym Centrum Matematycznym im. Stefana Banacha w Warszawie, w ramach Semestru ze Statystyki Matematycznej.This is a set of lectures delivered at the International Mathematical Centre, Warsaw, in 1976. The text contains three sections. Section 1 is devoted to basic notions. In the second section inference from a family of Poisson processes is considered. The last section deals with the problems of filtering of Cox processes (doubly stochastic processes)