פעילויות השבוע

Level raising theorems for modular forms are theorems about congruences of modular forms between different levels. These theorems play an important role in the proof of the Fermat’s last theorem by Wiles. In this talk, we will report some recent work on realizing level raising theorems for automorphic forms on GSp(4) by studying the geometry of certain quaternionic unitary Shimura variety.

We introduce a combinatorial method of generating random submanifolds of a given manifold in all dimensions and codimensions. The method is based on associating random colors to vertices, as in recent work by Sheffield and Yadin on curves in 3-space. We determine conditions on which submanifolds can arise in which ambient manifolds, and study the properties of random submanifolds that typically arise. In particular, we investigate the knotting of random curves in 3-manifolds, and discuss some other applications.

Joint work with Joel Hass

ניתן להגדיר (אי) אמנביליות של חבורה בדרכים רבות. בהרצאה נגדיר אי-אמנביליות של חבורה בדרך קומבינטורית, באמצעות חלוקות פרדוקסליות. חלוקה פרדוקסלית של חבורה היא חלוקה של החבורה למספר סופי של חלקים זרים כך שרק באמצעות הזזות שלהם (באמצעות איברים מהחבורה) ניתן לקבל שני עותקים של החבורה כולה. חבורה שקיימת לה חלוקה פרדוקסלית נקראת חבורה לא אמנבילית. מספר טרסקי של חבורה לא אמנבילית הוא מספר החלקים המינימלי בחלוקה פרדוקסלית שלה.

בהרצאה נביא דוגמאות לחבורות אמנביליות ולא אמנביליות ונוכיח כי קיימות חבורות לא אמנביליות עם מספר טרסקי גדול כרצונינו.

Szemeredi‘s theorem asserts that in every subset of the natural numbers of positive density one can find an arithmetic progression of arbitrary length. In 2001, Gowers gave a quantitative proof for this theorem. A key definition in his work are the Gowers norms which measure the randomness of subsets of the natural numbers. Inspired by Furstenberg‘s ergodic theoretical proof of Szemeredi‘s theorem, Gowers proved the following dichotomy: Either the given set is close to a random set with respect to these norms, or it admits some algebraic structure. Gowers then proved that in each of these cases Szemeredi‘s theorem holds. Later, Host and Kra studied the structure of certain ergodic systems associated with an infinitary version of the Gowers norms. Inspired by their work, Green, Tao and Ziegler improved Gowers‘ structure theorem showing that a function (or a set) with large Gowers norm must correlate with a nilsequence. This result is known as the inverse theorem for the Gowers norms. Recently, Jamneshan and Tao proved (roughly speaking) that a generalization of the Host-Kra theorem for ergodic systems associated with actions of the largest countable abelian group $\mathbb{Z}^\omega$ will imply the most general version of the inverse theorem for the Gowers norms. In this talk I will survey the above in more detail and mention some recent developments about these structure theorems.