In the third edition, on page 9, I am reading about the Trichotomy law.(adsbygoogle = window.adsbygoogle || []).push({});

It says, for every number a, one and only one of the following properties holds

(i) a = 0

(ii) a is in the collection P,

(iii) -a is in the collection

Before stating this, though, the author said that P is the collection of all positive numbers (set of positive numbers). If P is the set of positive numbers, how can negative a be in P?

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# Calculus by Spivak Trichotomy Law

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