Regular decomposition and a framework of order reduced methods for fourth order problems

This paper is devoted to the construction of order reduced method of fourth order problems. A constructive framework is presented such that a problem on the high-regularity space can be deduced to an equivalent system on three low-regularity spaces which are connected by a regular decomposition corresponding to a decomposition of the regularity of the high order space. The generated numerical schemes based on the deduced problems can be of lower complicacy, and the framework is fit for various fourth order problems. Three fourth order problems are then discussed under the framework, including one in two dimension and two in three dimension. They are each corresponding to a regular decomposition, and thus are discretised based on the discretised analogues of the regular decomposition; optimal error estimates are given.