The abelian sandpile and related models

The Abelian sandpile model is the simplest analytically tractable model of self-organized criticality. This paper presents a brief review of known results about the model. The abelian group structure allows an exact calculation of many of its properties. In particular, one can calculate all the critical exponents for the directed model in all dimensions. For the undirected case, the model is related to q= 0 Potts model. This enables exact calculation of some exponents in two dimensions, and there are some conjectures about others. We also discuss a generalization of the model to a network of communicating reactive processors. This includes sandpile models with stochastic toppling rules as a special case. We also consider a non-abelian stochastic variant, which lies in a different universality class, related to directed percolation.Comment: Typos and minor errors fixed and some references adde

The Economics of Mega-projects

Mega-projects play a foremost role not only in developed countries, but mainly in developing countries in several respects: political dynamics, so-cial effects, and economic fallout (see Bovensiepen and Meitzner Yoder, 2018). As correctly stated by van Wee and Tavasszy (2008), first, they are heavily under debate at the political level because of their economic impacts and important budget implications . Second, there is a lively debate also among scholars and practitioners owing to the huge cost escalation and schedule delay of these projects (Flyvbjerg et al., 2003a; Odeck, 2004), but also due to the uncertainty of their wider economic effects, which can be in-terpreted as related externalities

Cell motility as random motion: A review

The historical co-evolution of biological motility models with models of Brownian motion is outlined. Recent results for how to derive cell-type-specific motility models from experimental cell trajectories are reviewed. Experimental work in progress, which tests the generality of this phenomenological model building is reported. So is theoretical work in progress, which explains the characteristic time scales and correlations of phenomenological models in terms of the dynamics of cytoskeleton, lamellipodia, and pseudopodia