How far is to the next recurrent configuration? An NP-complete problem in the sandpile model.

Matthias Schulz How far is to the next recurrent configuration? An NP-complete problem in the sandpile model. 253-269 2008 Automata conf/automata/2008 db/conf/automata/automata2008.html#Schulz08 http://uncomp.uwe.ac.uk/free-books/automata2008reducedsize.pdf