Nonlinear Belief Change

This thesis concerns belief change without linearity, main contribution includes:(1) We establishes a representation theorem for a partial meet contraction which is defined by a nonlinear relation and satisfies a complete condition.(2) Based on lattice theory, we introduce a relation called LEE (Lattice- Epistemic Entrenchment), explore the properties of contraction based on LEE and get a serial of interesting representation theorems.(3) We present a relation called REE, and investigate systematically the relation between LEE and REE. Furthermore, we show that there exists a nonlinear LEE relation. Finally, we discuss how to represent nonlinear LEE by linear LEEs.