dear readerour aim with the series simula springerbriefs on computing is to provide compact introductions to selected fields of computing entering new field of research can be quite demanding for graduate studentspostdocsand experienced researchers alikethe process often involves reading hundreds of papersand the methodsresults and notation styles used often vary considerablywhich makes for time-consuming and potentially frustrating experience the briefs in this series are meant to ease the process by introducing and explaining important concepts and theories in relatively narrow fieldand by posing critical questions on the fundamentals of that field typical brief in this series should be around pages and should be well suited as material for research seminar in well-defined and limited area of computing we have decided to publish all items in this series under the springeropen frameworkas this will allow authors to use the series to publish an initial version of their manuscript that could subsequently evolve into full-scale book on broader theme since the briefs are freely available onlinethe authors will not receive any direct income from the saleshoweverremuneration is provided for every completed manuscript briefs are written on the basis of an invitation from member of the editorial board suggestions for possible topics are most welcome and can be sent to aslak@simula no january prof aslak tveito ceo dr martin peters executive editor mathematics springer heidelberggermany vii