Reductions of Operator Pencils
For mathematicians working on operator theory and differential equations, this provides a theoretical framework for pencil reduction, but the results are incremental and abstract without concrete applications or numerical results.
The paper studies reductions of operator pencils on Banach spaces, proving that two natural reduction operations commute under mild assumptions. The pivot operators from these reductions are linked to regular pencils in finite dimensions and the inf-sup condition for saddle point problems, and are used to describe the generalized spectrum.
We study problems associated with an operator pencil, i.e., a pair of operators on Banach spaces. Two natural problems to consider are linear constrained differential equations and the description of the generalized spectrum. The main tool to tackle either of those problems is the reduction of the pencil. There are two kinds of natural reduction operations associated to a pencil, which are conjugate to each other. Our main result is that those two kinds of reductions commute, under some mild assumptions that we investigate thoroughly. Each reduction exhibits moreover a pivot operator. The invertibility of all the pivot operators of all possible successive reductions corresponds to the notion of regular pencil in the finite dimensional case, and to the inf-sup condition for saddle point problems on Hilbert spaces. Finally, we show how to use the reduction and the pivot operators to describe the generalized spectrum of the pencil.