Erica Jen, the first postdoctoral fellow and former Deputy Director of the Center for Nonlinear Studies (CNLS), passed away on November 12, 2023, at 71. She was held in high esteem by all who knew her for her ability to bring together people from diverse backgrounds and disciplines and to foster interdisciplinary collaborations, broadening research scopes and inspiring diverse perspectives in complex adaptive systems.

Erica was an outstanding mathematician born in 1952 to Chih-Kung Jen, a Chinese-born physicist who played a significant role in reestablishing scientific exchanges between the U.S. and China in the 1970s. While a young undergraduate at Yale, Jen accompanied her father, theoretical physicist Chih-Kung Jen, to China and asked the then-Chinese Premier, Zhou Enlai, if she could stay. She became one of the first Americans allowed to study in China after the Cultural Revolution.

Her experiences studying at Beijing University and working in physical labor roles during a politically turbulent time in China profoundly influenced her career trajectory. She once said, "China taught me a lot about the effectiveness of bottom-up/top-down decision-making, the need for equity in the workplace, and the importance of having an open atmosphere in which you encourage criticism and self-criticism." These experiences were described in some detail by her fellow U.S. student in China, Jan Wong, in her book "Red China Blues."

She returned to the U.S. and obtained her Ph.D. in applied mathematics from Stony Brook, New York, before becoming a CNLS postdoc. In 1983-86, she served as a university scholar and assistant professor of mathematics at the University of Southern California. She was supported in this move to academia by CNLS Advisor Mark Kac, who was very interested in and impressed by her work in mathematics. In 1986, she returned to Los Alamos to become a staff member in the theoretical division and acting deputy director of the CNLS.

Her research in the mathematical analysis of chaotic and complex behavior included the singular integral equations for crack propagation, the mathematics of cellular automata - a class of dynamical systems that evolve according to simple local interaction rules - and mechanisms of robustness in natural and social systems.

While at LANL, she developed some of the first mathematical foundations for understanding cellular automata. These systems are defined by "rule tables" (e.g., lookup tables); not much traditional mathematics applies to such systems. In this sense, cellular automata are the simplest possible prototypes for the more complicated systems with arbitrary or random interconnections, heterogeneous agents, or stochasticity in their interactions. They are now widely used for simulation studies of behavior such as fluid flows, percolation through porous media, traffic congestion, bird flocking and navigation, species extinctions, and many other natural and social sciences phenomena.

At first, the cellular automata were studied exclusively through simulations. Her focus was to create a theoretical framework to answer questions, such as when two cellular automata that have different interaction mechanisms and that generate different-looking behavior are, in fact, equivalent under some hidden mathematical transformation. Or that a particular cellular automaton will eventually evolve to a state consisting of spiral waves regardless of its beginning state: it's impossible to establish that kind of result with simulations since it would require trying all possible initial conditions and letting the systems evolve for infinite time.

Erica was known for her focus on problem formulation and mechanisms. She was not afraid to explore unconventional or "flaky" approaches to problem-solving, challenging traditional definitions of problems and solutions. Her insights led to the development of powerful metaphors that changed how people approached their research. She actively sought collaborations across disciplines to tackle common topics, utilizing diverse tools and methods to address aspects of problems often overlooked in traditional disciplinary frameworks. She epitomized this approach through her involvement with CNLS, often acting as a counselor to young researchers and being very much the conscience of CNLS through her selfless efforts on its behalf.

In 1995, she joined the Santa Fe Institute (SFI) and served as vice president for academic affairs and later as a research professor. While at Santa Fe Institute, she developed innovative programs in evolutionary dynamics, social networks, and distributed learning. She was the principal investigator for the groundbreaking SFI program on the robustness of physical, biological, ecological, and social systems.

After several years at the SFI, Erica faced a significant personal challenge when her daughter "Po" was born with a serious, and usually mid-term fatal, birth defect. Erica was undeterred and poured her intense energy and focus into caring for Po, first communicating with her by sign language and then, to the amazement of skeptical physicians, teaching her how to speak and read lips to communicate with others. Erica is survived by Po and her husband, George Zweig.

Erica understood the differences between toy problems and real problems and used simulations to connect theory and the real world. In an interview at SFI, she said,
"Simulation is often thought of as being somewhere in between and is even seen by some as closer to experiment since the computer generates results that look like data. But that, of course, is scary since there's all the difference between computer results and actual data. If data are what scientists use in at least some types of problems to keep them honest, and if it starts to get confusing about what constitutes data.

The problem is that as the scale and complexity of our computer simulations increase, so does our inability, in many cases, to make sense of those simulations. We lack even the conceptual tools to consider the reliability and scientific validity of many of our most intriguing simulations.

But people here and elsewhere are beginning now to think about developing a science of simulation. And personally I find that one of the most promising areas of scientific research for the Institute: we've got a head start on the problem, and the payoffs are enormous. Computer simulations are the wave of the future."

This philosophy was reflected in an article she wrote with several CNLS colleagues entitled "Experimental Mathematics: The Role of Computation in Nonlinear Science", published in the Communications of the Association of Computer Machinery 28, 374-384, 1989. Erica took particular delight in the seeming oxymoron of the title "experimental mathematics" and its shocking effect on her mathematical colleagues. Still, history has proven her perspective to be accurate, and many nonlinear phenomena have been understood in detail only after careful, well-conceived computations have revealed crucial insights.

Erica connected mathematical and cultural communities, uniting individuals to gain a deeper understanding of the intricacies of our world. All who had the privilege of knowing her will deeply feel her absence.