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Lectures on invertible field theories

Four lectures on invertible field theories at the Park City Mathematics Institute 2019. Cobordism categories are introduced both as plain categories and topologically enriched. We then discuss localization of categories and its relationship to classifying spaces, and state the main theorem of classification of invertible field theories in these terms. We also discuss symmetric monoidal structures and their relationship to actions of the little disk operads. In the final lecture we discuss an application of cobordism categories to characteristic classes of surface bundles. Emphasis will be on self-contained definitions and statements, referring to original literature for proofs. We also include problem sets from the three exercise sessions at the summer school, and solutions to two problems.