The inverse of differential operators and an extension of Trefftz method (Q2717036)

The purpose of this paper is to present an extension of Trefftz method. The essence of the proposed approach is to obtain lower bounds on eigenfrequencies of elastic continuous structures, if we have good upper bounds calculated by a simple method. Secondly, it is possible to examine some problems related to the inverses of differential operators. These procedures essentially lead to the construction of Green matrices. In the first part of the appendix a short summary of the results connected to the generalized Green matrix is given, and technique is presented in an example for constructing the generalized Green matrix.