Correspondence item: Stability, instability and aperiodicity tests for linear discrete systems

The system characteristic polynomial is replaced by its inverse which is decomposed as a combination of lower-degree and lower-degree inverse polynomials. A sequence of polynomials of descending degree is determined by successive decomposition. A necessary and sufficient condition of stability as well as a sufficient condition of instability, depending on the coefficients of decomposition, are given. The test for stability or instability is proposed. When testing aperiodicity, the transformation mapping the real segment (0, 1) onto the periphery of the unit circle, is used. The system characteristic polynomial whose roots are to be tested for aperiodicity is replaced by another one whose roots should be tested for stability.