A simpler construction of the matrix chain defining the tractability index of linear DAEs

This letter introduces a new characterization of the tractability index for linear differential–algebraic equations (DAEs). The matrix function chain defining the index is built in a simpler manner which relies upon invariant spaces. In particular, the construction can be easily performed in terms of orthogonal projectors. A clearer characterization of the tractability index results, and a simple description of the Kronecker index is obtained for the constant coefficient case. An example coming from circuit theory illustrates the new construction.