皆様，
東北大学の横山です． 以下の通りセミナーを開催いたしますのでご案内いたします．
https://sites.google.com/view/sendai-logic/
日時：6月9日(金)15:00〜 場所：東北大学理学研究科合同A棟801号室 (zoom配信あり)
講演者：Paul Shafer (University of Leeds)
Title: The Rival--Sands theorem for partial orders, ascending/descending sequences, and Sigma_2 induction.
Abstract: In their 1980 article, Rival and Sands proved two theorems inspired by Ramsey’s theorem for pairs. The second of these theorems states that every infinite partial order P of finite width contains an infinite chain C with the property that every element of P is comparable either with no element of C or with infinitely many elements of C. We show that this theorem is equivalent to the ascending/descending sequence principle plus the Sigma_2 induction scheme over RCA_0. This work is joint with Marta Fiori-Carones, Alberto Marcone, and Giovanni Soldà.
どうぞよろしくお願いいたします．
横山啓太 -- Keita Yokoyama Mathematical Institute, Tohoku University Aoba, Sendai, JAPAN, 980-8578 [email protected]