In the Spring 2027 semester the New York Group Theory Seminar will meet with most talks in-person and some occasional talks online. The in-person talks will be on Fridays at 4:15pm eastern time, room TBA. The online Zoom talks will be on Fridays at 4:00pm U.S. eastern time.
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New York Group Theory Seminar: Friday, January 29, 2027, 4:15pm, room TBA, CUNY Graduate Center
Speaker: Jayadev Athreya (Fordham University)
Title: The quasi-isometry classes of Galton--Watson trees
Abstract:
This is joint work with Sascha Troscheit (Uppsala) on the large-scale geometry of Galton–Watson trees, a classical model of rooted random trees, where each vertex has an independent number of children (chosen according to an offspring distribution). We will explain what Galton-Watson trees are, so no familiarity with the model is assumed. We give a classification of offspring distributions into classes such that two independent trees almost surely admit a root-preserving quasi-isometry when their offspring distributions belong to the same class and are almost surely not quasi-isometric when they belong to different classes. We will describe this classification, and some ideas behind the proof in the important special case where each vertex has one or two children.
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New York Group Theory Seminar: Friday, February 19, 2027, 4:15pm, room TBA, CUNY Graduate Center
Speaker: Eamonn Olive (CUNY Grduate Center)
Title:
Abstract:
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New York Group Theory Seminar: Friday, February 26, 2027, 4:15pm, room TBA, CUNY Graduate Center
Speaker: Alexander Hoffmann (Hunter College)
Title: Geodesics in small cancellation groups
Abstract:
Let G be a group characterized by a small cancellation $C'(1/8)$-presentation. We study the structure of its Cayley graph, specifically geodesic intervals and digons. We prove that all geodesic segments [x,y] of length $n$ can be covered by finitely many digons with pairwise $C'(1/8)$-short lower intersections whose lower sides lie on a common geodesic segment [x,y]. This is a generalization of a proposition of Kharlampovich-Sklinos for random groups. We then bound this resulting diagram of digons, showing that the total perimieter of its cells and its edge count is bounded by $n$, and the cycle rank is equal to the number of distinct cell contours. Thus, although there may be many different geodesics, the size of their union is bounded linearly in the distance between their endpoints. The constant is independent of the number and lengths of the defining realtors. In particular, the estimate applies to infinite presentations with unbounded relator lengths. From this we get a consequence for growth, allowing us to compare the number of geodesic words with the number of group elements of the same length.
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