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Single Idea 15904

[filed under theme 6. Mathematics / A. Nature of Mathematics / 3. Nature of Numbers / i. Reals from cuts ]

Full Idea

Roughly speaking, the upper and lower parts of the Dedekind cut correspond to the commensurable ratios greater than and less than a given incommensurable ratio.

Clarification

'Commensurable' implies 'knowable'

Gist of Idea

The two sides of the Cut are, roughly, the bounding commensurable ratios

Source

Shaughan Lavine (Understanding the Infinite [1994], II.6)

Book Ref

Lavine,Shaughan: 'Understanding the Infinite' [Harvard 1994], p.38


A Reaction

Thus there is the problem of whether the contents of the gap are one unique thing, or many.


The 11 ideas with the same theme [defining real numbers by cutting the line of rationals]:

A cut between rational numbers creates and defines an irrational number [Dedekind]
I say the irrational is not the cut itself, but a new creation which corresponds to the cut [Dedekind]
Dedekind's axiom that his Cut must be filled has the advantages of theft over honest toil [Dedekind, by Russell]
Dedekind says each cut matches a real; logicists say the cuts are the reals [Dedekind, by Bostock]
A series can be 'Cut' in two, where the lower class has no maximum, the upper no minimum [Russell]
A real number is the class of rationals less than the number [Russell/Whitehead, by Shapiro]
Points are 'continuous' if any 'cut' point participates in both halves of the cut [Harré/Madden]
For Eudoxus cuts in rationals are unique, but not every cut makes a real number [Bostock]
Why should a Dedekind cut correspond to a number? [Fine,K]
Cuts are made by the smallest upper or largest lower number, some of them not rational [Shapiro]
The two sides of the Cut are, roughly, the bounding commensurable ratios [Lavine]