What makes this gap so persistent is the nature of the characteristic equation itself. For an ordinary differential equation without delay, stability can be checked with the Routh–Hurwitz criterion, a finite algebraic test on the coefficients. Introduce even a single delay, however, and the characteristic equation becomes transcendental: the exponential term means there are infinitely many roots, and their collective behavior must be controlled as the coefficients vary. A root that sits safely in the left half of the complex plane at one parameter value can migrate across the imaginary axis at another, and because the roots form an infinite family, no finite checklist of crossings suffices in general. This is precisely why the literature has accumulated so many partial results—criteria valid under sign restrictions on the coefficients, or for special sub-cases obtained by setting certain parameters to zero—while the full four-parameter problem resisted a definitive treatment.

The approach taken by the CICESE researchers builds on a classical pillar of the field: Pontryagin’s theorem on quasi-polynomials. Quasi-polynomials are expressions that combine polynomial terms with exponentials, and they arise naturally as characteristic functions of delay differential equations. Pontryagin’s result provides conditions under which the zeros of such functions can be controlled, effectively reducing an infinite-dimensional spectral question to a finite geometric one. What the new work adds is a novel geometric layer: rather than examining each candidate crossing curve separately and counting roots, the authors define a family of parametric curves and take their supremum envelope. The envelope collapses the potentially complicated crossing geometry into a single computable object whose epigraph—everything above the curve—is exactly the stable region.

This constructive character is what separates the contribution from purely numerical alternatives. Methods such as pseudospectral discretization of the solution operator, or large-scale quasi-polynomial root mapping, approximate the rightmost characteristic roots efficiently and are invaluable for large systems, but they deliver answers at specific parameter points rather than a global picture. The cluster treatment of characteristic roots and frequency-sweeping tests similarly determine, for fixed coefficients, the set of admissible delays. The new result inverts the question: for a fixed delay, it describes the entire set of stabilizing coefficients at once. The two viewpoints are related by a scaling map—varying the delay traces a ray through parameter space, and stability holds exactly where that ray intersects the stable region—so delay margins can be recovered from the coefficient-space picture, but the inversion is what makes gain charts and robustness assessment natural.

The practical payoff of having an explicit boundary is considerable. Because the stable region is an epigraph with a computable boundary curve, robustness indices follow immediately as distances from a nominal operating point to that boundary. In control design, such margins quantify how much parameter drift—a change in gain, a shift in actuator latency, wear in a mechanical component—a closed-loop system can tolerate before losing stability. When the boundary must be found numerically, engineers typically sample and interpolate; here the boundary is available in closed parametric form, so the distance computation inherits the precision of the underlying formulas. The accompanying algorithm supports standard visualization workflows, meaning the stability landscape can be plotted, inspected, and used interactively during controller tuning.

The examples in the paper illustrate how the general characterization specializes. For structured sub-classes of the four-parameter family, the envelope description collapses to very simple closed-form stability criteria—inequalities that can be checked by hand. One such example concerns an internal combustion engine model, for which the authors derive a delay-independent stabilization condition: a set of coefficient relationships under which the system remains stable for every value of the delay, no matter how large. Delay-independent stability is a strong and desirable property in applications where the delay is uncertain or time-varying, since it removes the delay from the list of quantities that must be bounded or measured. That such a condition emerges as a corollary of the general theory, rather than requiring a separate ad hoc analysis, demonstrates the unifying power of the characterization.

It is worth appreciating which parts of the construction are classical and which are genuinely new. The parametrization of the imaginary-axis crossing locus, the invariance of the number of unstable roots on connected components of the complement of the crossing curve, and the computation of crossing directions via eigenvalue perturbation are all standard elements of the D-partition method, which dates back to mid-twentieth-century work on automatic control. The new ingredients are threefold: first, the identification of the stable component of the crossing geometry without having to perform a root count at a test point in every component; second, the closed-form description of that component as the epigraph of the supremum envelope; and third, a global geometric theorem showing that the crossing curve admits no simple counterclockwise loop, which allows a local instability observation to be propagated to every non-stable component and thereby delivers necessity of the conditions. Necessity is the hard part—sufficient conditions are comparatively easy to obtain, but proving that every point outside the described region is genuinely unstable is what elevates the result from a useful criterion to a complete characterization.

The broader context of delay-differential equations helps explain why second-order systems with a single delay occupy such an important niche. Retarded functional differential equations, in which the rate of change depends on past states, model phenomena ranging from population dynamics with gestation periods to cutting forces in machining that depend on the tool position one revolution earlier. In feedback control, delays arise from actuation latency, sensor processing, transport of material or information, and computation time. Even when the underlying plant is linear, the delay makes the dynamics infinite-dimensional, and phenomena such as stability lobe diagrams—familiar to machinists as the lobed boundaries separating stable from chattering cutting conditions—emerge from the interplay between the delay and the gains. The classical stability lobe analysis for the undamped sub-case of the second-order equation is one of the special cases that the new theorem subsumes, placing a graphical technique used in manufacturing for decades within a single rigorous framework.

The paper also situates itself within a decade of intense activity on delay-system stability. Time-domain methods based on the delay Lyapunov matrix have matured to the point of delivering necessary and sufficient tests, and frequency-domain criteria continue to be refined. A distinct recent line of research exploits multiplicity-induced dominancy—the observation that characteristic roots of maximal multiplicity in low-order retarded equations tend to be the dominant ones—to assign the spectral abscissa directly. In the PID-stabilization literature, generalized Hermite–Biehler arguments have characterized stabilizing gains for low-order plants, and pseudoconcavity arguments have yielded the exact delay margin achievable by PID control of second-order systems. These strands, however, typically optimize along the delay axis for a structured controller or treat restricted parameter sub-classes; the coefficient-space region for the full four-parameter class had remained open.

For practitioners, the significance lies in the transition from pointwise testing to global design. A gain chart produced by the method shows, for fixed delayed-channel parameters, the complete region of stabilizing local parameters and its boundary, rather than a scatter of tested points. This means a designer can see at a glance how much freedom remains after choosing an operating point, where the nearest instability lies, and how the region deforms as the delay or the delayed-feedback gains change. Because the region is connected—being an epigraph—there are no isolated stable islands to hunt for, which simplifies both computation and intuition. The authors suggest that the framework supports standard control-design workflows directly, from visualization to margin computation to tuning.

The mathematical community’s interest in complete low-dimensional characterizations is not merely academic. Low-order delay systems serve as canonical models in vibration suppression, human balance control, and networked control systems, and they are the building blocks from which intuition about higher-order and multi-delay problems is formed. A definitive answer in the single-delay second-order case provides a benchmark against which numerical methods can be validated and a template that may guide extensions to multiple delays or higher order, where the geometric complexity of the crossing curves grows substantially. The supremum-envelope technique, in particular, offers a way of organizing complicated boundary geometry into a single monotone object, and whether analogous envelopes exist for richer classes of quasi-polynomials is a natural question for further work.

Finally, the result underscores a recurring theme in control theory: that classical tools, when combined with the right geometric insight, can resolve problems that computational methods alone leave only approximately answered. Pontryagin’s theorem has been available since the 1940s, and the D-partition technique since the 1950s, yet the complete four-parameter characterization required identifying the correct envelope construction and proving the global loop-free property that makes necessity tractable. The outcome is a rare thing in the stability theory of delay equations—a closed-form, necessary and sufficient answer—and it arrives with the algorithmic machinery to make it usable in practice.

Subject of Research: Research on the complete stability region characterization for second-order single-delay linear systems.

Article Title: Complete characterization of the asymptotic stability region for single-delay second-order linear systems

Article References: Cuesta-García, J. R., & Márquez-Martínez, L. A. (2026). Complete characterization of the asymptotic stability region for single-delay second-order linear systems. Frontiers in Control Engineering, 7, Article 1885855. https://doi.org/10.3389/fcteg.2026.1885855

Image Credits: AI Generated

DOI: 10.3389/fcteg.2026.1885855

Keywords: time-delay systems, asymptotic stability, control theory, quasi-polynomials, stability region, retarded functional differential equations, Pontryagin theorem, robustness margins, Complete, characterization, asymptotic, stability