Celso Grebogi investigated strange nonchaotic attractors with Edward Ott, Steven Pelikan and James A. Yorke. Their 1984 construction separated geometrical irregularity from exponentially growing trajectory differences.[ref]
Strange attractors that are not chaotic by Celso Grebogi and others. Examples and proof of transverse contraction
A quasiperiodically driven map can develop increasingly fine structure while retaining a negative transverse Lyapunov exponent. The neutral drive direction must be distinguished from that transverse response. This provides a concrete counterexample to identifying every fractal-looking attractor with chaos.
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Celso Grebogi is a Brazilian physicist, known for his work in the fields of dynamical systems, chaos theory, and control systems. He has made significant contributions to understanding complex systems and nonlinear dynamics, particularly in areas related to chaos control and synchronization. Grebogi is also recognized for his research in applying mathematical concepts to practical problems, including those in engineering and biology. Beyond his academic contributions, he has been involved in various collaborative projects and has published numerous research papers throughout his career.