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        "event_title":"Student Model Theory Seminar (Invited Address)",
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        "combined_title":"Student Model Theory Seminar (Invited Address): Ronnie Chen",
        "event_subtitle":"Ronnie Chen",
        "event_type":"Lecture \/ Discussion",
        "event_type_id":"13",
        "description":"Title: An introduction to infinitary first-order (and propositional) logic\r\n\r\nAbstract: What happens to first-order logic if you allow the logical connectives and\/or quantifiers to be infinitely long? It turns out that many of the standard tools and results (e.g., compactness) either break or trivialize, while new interesting phenomena arise. Indeed, infinitary propositional logic is already quite interesting, and is closely related to (in some sense, equivalent to) topology. Moreover, the interaction between propositional and first-order plays a central role in infinitary logic, and leads to the application of tools from topology and group actions to model theory. This talk will survey a proper subset of these topics, such as the Scott isomorphism theorem, omitting types theorem, and Lopez-Escobar theorem.",
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        "tags":["Graduate Students","Mathematics","seminar","Talk","Undergraduate Students"],
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    {
        "datetime_modified":"20250420T111715",
        "datetime_start":"20250421T160000",
        "datetime_end":"20250421T170000",
        "has_end_time":1,
        "date_start":"2025-04-21",
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        "time_zone":"America\/Detroit",
        "event_title":"The Twist for Positroid Varieties, Part 2",
        "occurrence_title":"",
        "combined_title":"The Twist for Positroid Varieties, Part 2: Yucong Lei",
        "event_subtitle":"Yucong Lei",
        "event_type":"Workshop \/ Seminar",
        "event_type_id":"21",
        "description":"Last week we introduced positroid varieties and their descriptions through bounded affine permutations and equivalently Grassmann necklaces, and talked about some relevant plabic graph combinatorics. In this talk, we pick up from the boundary measurement map to give a parametrization of points in the positroid. We will then introduce the twist automorphism, which will connect the image of the boundary measurement map to the nonvanishing information given by face labels of a plabic graph. We will then give a combinatorial way of computing twisted face pluckers from \"minimal matchings\" of the plabic graph, which will turn out to be invertible, hence showing the boundary measurement map is an open inclusion into the positroid variety over complex numbers.",
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