I will give an introduction
to the ``abelian sandpile model" (ASM).
In this model to every vertex i of
a finite subset V of Zd we associate a integer valued
height variable hi>0 (``the number of grains at i").
If hi < 2d+1 the site is called stable. An unstable
configuration is stabilized by a sequence of ``topplings"
consisting of removing 2d grains from unstable
sites and giving one grain to each neighbor. In the ASM dynamics,
one picks a vertex at random, adds a grain and stabilizes
The recurrent configurations of ASM form an abelian group (under pointwise addition and stabilization). They can also be characterized by an algorithm which gives a bijection to the set of rooted spanning trees on V. The effect of adding one grain to a recurrent configuration can be characterized by a sequence of ``waves", each of which can be represented by a two component spanning tree. Spanning trees are related to loop-erased random walk via Wilson's algorithm.
If time is left, I will discuss some recent results for the ASM on infinite graphs.
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