>>698 補足

検索 a countably infinite fair lottery
で、下記ヒット

正直むずい
ノンスタ使って、うんぬんかんぬん

でも、”1/2 by symmetry”は出てこなかったので
ダメみたいですね

(参考)
https://philarchive.org/archive/WENFIL
Synthese DOI 10.1007/s11229-010-9836-x
Fair infinite lotteries Sylvia Wenmackers · Leon Horsten

Received: 2 September 2010 / Accepted: 14 October 2010 ©TheAuthor(s) 2010. This article is published with open access at Springerlink.com

Abstract
This article discusses how the concept of a fair finite lottery can best be extended to denumerably infinite lotteries. Techniques and ideas from non-standard analysis are brought to bear on the problem.

Keywords Foundations of probability · Non-standard analysis · Countable additivity · Infinity
1 Introduction: from the finite to the transfinite