Compactness under constructive scrutiny

✍ Scribed by Hajime Ishihara; Peter Schuster

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 176 KB
• Volume: 50
• Category: Article
• ISSN: 0044-3050
• DOI: 10.1002/malq.200310119

✦ Synopsis

Abstract

How are the various classically equivalent definitions of compactness for metric spaces constructively interrelated? This question is addressed with Bishop‐style constructive mathematics as the basic system – that is, the underlying logic is the intuitionistic one enriched with the principle of dependent choices. Besides surveying today's knowledge, the consequences and equivalents of several sequential notions of compactness are investigated. For instance, we establish the perhaps unexpected constructive implication that every sequentially compact separable metric space is totally bounded. As a by‐product, the fan theorem for detachable bars of the complete binary fan proves to be necessary for the unit interval possessing the Heine‐Borel property for coverings by countably many possibly empty open balls. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)