A General Framework for Upper and Lower Possibilities and Necessities
We show that a (fuzzy) multivalued mapping carries a possibility and necessity measure defined over (fuzzy) subsets of a first universe into a system of upper and lower possibilities and necessities defined over (fuzzy) subsets of a second universe. Upper possibilities (resp. lower necessities) form again a possibility measure (resp. necessity measure), while lower possibilities and upper necessities both form a confidence measure. The approach presented is based on possibilistic conditioning, in particular on the definitions of conditional possibilities and necessities in a general framework based on triangular norms and conorms. In case of a multivalued mapping, upper and lower possibilities and necessities can be expressed equivalently in terms of conditional possibilities and necessities of lower and upper inverse images under the given multivalued mapping, or in terms of a basic possibility assignment and a basic necessity assignment. In case of a fuzzy multivalued mapping, such equivalent expressions cannot be established in general. Upper and lower possibilities and necessities can then be introduced in two alternative ways. For normalized fuzzy multivalued mappings or crisp multivalued mappings, interesting relationships are obtained.