A formal framework for SMP is important because it facilitates the building of algorithm model and the evaluation of algorithms. First, we propose a formal definition of schema matching that is named multivalent matching, i.e., an individual of source schema can be associated with a set of individuals of target schema. Second, we develop the algebraic framework of multivalent matching that is called schema homomorphism and propose the algorithm model of SMP. Then, we discuss the relations between match cardinality and variants of schema homomorphism.