Preface of STACS 2013 Special Issue

This special issue contains six articles based on extended abstracts that were presented at the 30th Symposium on Theoretical Aspects of Computer Science (STACS), which was held at the Christian-Albrechts-Universitat zu Kiel, Germany, from February 27th to March 2nd, 2013. These extended abstracts were among the top papers of those chosen for presentation at STACS 2013 in a highly competitive peer-review process: the members of the program committee with chairs Thomas Wilke and Natacha Portier selected only 54 papers out of 254 submissions. Compared with the original extended abstracts that appeared in the conference proceedings the articles in this issue have been extended by full proofs and additional results. They underwent a further rigorous reviewing process, following the TOCS standard, completely independent of the selection process of STACS 2013. The paper The Arithmetic Complexity of Tensor Contraction considers the fundamental arithmetic complexity class VP. This class was originally defined by Valiant, in the framework of his theory of arithmetic complexity, as the class of sequences of polynomials with polynomially bounded degree that can be computed by polynomially sized circuits. Although of quite some interest, as the analog of deterministic polynomial time in arithmetic complexity, this class is not too well understood so far. This is partly due to the fact that no really natural characterization of it has been known. In the paper polynomials in VP are characterized as functions obtainable as