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Tuesday, September 11, 2018

**Abstract:** This is a talk about an algebraic notion of a plethory. A plethory P determines a category of "P-rings", objects of which are commutative rings R equipped with a collection of functions $f_i : R \to R$ satisfying a list of axioms. Many interesting cohomology theories take values in a category of P-rings for some plethory P. The motivating example is K-theory, which takes values in "Lambda-rings", which is precisely the category of rings for the Lambda plethory. This talk will be expository, concentrating first on interesting examples of P-rings, then working backward to the definition of plethory. Then I'll talk about the "Witt ring" construction associated to any plethory, which includes and generalizes the classical construction of "Witt vectors".