皆様，
東北大学の竹田です． 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：11月22日(金)16:00〜 場所：zoom配信
講演者：Giovanni Solda (Ghent University)
題目 A proof of Nash-Williams theorem in ATR0 概要 In [1], Crispin Nash-Williams proved that if (Q, ≤_Q) is a well quasi-order (henceforth wqo), then so is the set of transfinite sequences over Q with finite range, ordered by embeddability. In this talk, we will show that this result is provable in the subsystem of second-order arithmetic ATR_0: together with previous results by Shore [2], this determines the reverse mathematical strength of Nash-Williams’ result. In order to do this, we will go via the notion of better quasi-order, which makes it possible to develop an equivalence between the iterated powerset of a qo Q and the iterated powerset over Q. This is joint work with Fedor Pakhomov. [1] C. Nash-Williams, On well-quasi-ordering transfinite sequences, Mathematical Proceedings of the Cambridge Philosophical Society 61 (1965), no. 1, 33–39. [2] R. A. Shore, On the strength of Fraïssé’s conjecture, Logical methods: In honor of Anil Nerode’s sixtieth birthday, 1993, pp. 782–813.
参加を希望される方は竹田([email protected])宛にご連絡いただきますようよろしくお願いいたします.
どうぞよろしくお願いいたします．
竹田 侑人 [email protected]