\BOOKMARK [1][-]{section.1}{1. Introduction}{} \BOOKMARK [1][-]{section.2}{2. Preliminaries}{} \BOOKMARK [2][-]{subsection.2.1}{2.1. Conventions}{section.2} \BOOKMARK [2][-]{subsection.2.2}{2.2. Torsors and twists}{section.2} \BOOKMARK [2][-]{subsection.2.3}{2.3. CFG's, stacks and gerbes}{section.2} \BOOKMARK [2][-]{subsection.2.4}{2.4. Essential and canonical dimension of CFG's}{section.2} \BOOKMARK [1][-]{section.3}{3. Fibers for morphisms of CFG's}{} \BOOKMARK [1][-]{section.4}{4. p-exhaustive subgroups}{} \BOOKMARK [1][-]{section.5}{5. Canonical dimension of group schemes}{} \BOOKMARK [1][-]{section*.2}{Acknowledgments}{} \BOOKMARK [1][-]{section*.3}{References}{}