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Julia Robinson

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Julia Robinson was a prominent American mathematician. She made major contributions to computability theory and computational complexity theory. These fields study what can be calculated by a machine or a set of rules. She is most famous for her work on decision problems. A decision problem is a question that has a yes or no answer. Robinson played a crucial role in resolving a famous puzzle known as Hilbert's tenth problem. This work eventually led to the MRDP theorem, named after Matiyasevich, Robinson, Davis, and Putnam.

Robinson's mathematical journey began after overcoming significant personal hurdles. When she was nine years old, she suffered from scarlet fever and rheumatic fever. These illnesses forced her to miss two years of school. After she recovered, a private tutor helped her catch up quickly. She completed four years of school curriculum in only one year. She later attended San Diego State University at age 16. She eventually transferred to the University of California, Berkeley, for her senior year in 1939. At Berkeley, she studied number theory under Raphael M. Robinson, whom she later married.

After earning her BA in 1940, Robinson pursued graduate studies at Berkeley. She worked as a teaching assistant in the mathematics department and a statistics lab assistant for Jerzy Neyman. In 1948, she earned her PhD under Alfred Tarski. Her dissertation focused on "Definability and Decision Problems in Arithmetic." She proved that the theory of rational numbers is an undecidable problem. This means there is no universal method to solve all questions regarding them. She showed that elementary number theory could be defined using the arithmetic of rational numbers.

Robinson's most significant work involved Hilbert's tenth problem. This problem asks for an algorithm to determine if a Diophantine equation has integer solutions. Diophantine equations are polynomial equations where only integer solutions are sought. In 1948, while at the RAND Corporation, Robinson began exploring methods for this problem. By 1950, she developed the J.R. hypothesis, named after her. This hypothesis involved using Pell's equation and Diophantine representation for exponentiation. Proving this hypothesis was the key to solving the larger problem.

Her research led to long-term collaborations with other mathematicians. She worked with Martin Davis, Hilary Putnam, and Yuri Matiyasevich. In 1950, she met Davis, who was studying if all listable sets were Diophantine. Robinson was instead trying to show that specific sets, like prime numbers, were Diophantine. Starting in 1959, Robinson, Davis, and Putnam worked together on these ideas. They discovered that solutions to a specific "Goldilocks" equation were essential. In 1970, they finally resolved the problem in the negative. They proved that no such universal algorithm can exist.

Robinson also made important contributions to the field of game theory. During the late 1940s, she researched this subject at the RAND Corporation. In 1949, she published a report on the Hamiltonian game. This paper is credited as the first to use the term "travelling salesman problem." In 1951, she published a paper regarding an iterative method for solving games. She proved that fictitious play dynamics converge to the mixed strategy Nash equilibrium in two-player zero-sum games. This work addressed a prize problem posed by George W. Brown.

Despite her brilliance, Robinson faced professional obstacles at UC Berkeley. After marrying Raphael Robinson in 1941, a rule prevented family members from working in the same department. This meant she could not teach in the mathematics department. She worked in the statistics department instead, even though she wanted to teach calculus. It was not until 1976 that she was offered a full-time professorship. This offer came after her nomination to the National Academy of Sciences. She was the first female mathematician elected to that Academy in 1975.

Robinson's career was marked by many prestigious honors. She was the first female president of the American Mathematical Society from 1983 to 1984. In 1983, she received a $60,000 MacArthur Fellowship. She also became a member of the American Academy of Arts and Sciences. Robinson was dedicated to encouraging other women to become research mathematicians. She was diagnosed with leukemia in 1984 and died in 1985. Today, her legacy lives on through the Julia Robinson Mathematics Festival and various documentaries about her life.

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