2 Four Jewel – Problems About Borderlines

This is a list of research topics prepared on the occasion of the Fourth Brainstorming Week on Membrane Computing, Sevilla, January 30 – February 3, 2006 (hence the title). The selection is subjective, the presentation is informal (in most cases, the precise formulation of the question to investigate is already part of the research task), the bibliographical information is partial (giving only a starting point for searching the literature; as usual, for a complete bibliography the reader should refer to [36]).