V : Number Systems and Set Theory
V : Number systems and set theory Any reasonable framework for mathematics should include the fundamental number systems which arise in the subject: 1. The natural numbers N (also known as the nonnegative integers). 2. The (signed) integers Z obtained by adjoining negative numbers to N. 3. The rational numbers Q obtained by adjoining reciprocals of nonzero integers to Z. 4. The real numbers R, which should include fundamental constructions like nth roots of positive rational numbers for an arbitrary integer n > 1, and also –1 –2 –k ⋅ ⋅ ⋅ all “infinite decimals” of the form b1 10 + b2 10 + … + bk 10 + … where each bi belongs to {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}. Up to this point we have tacitly assumed that such number systems are at our disposal. However, in both the naïve and axiomatic approaches to set theory it is eventually necessary to say more about them. The naïve approach. In naïve set theory it is necessary to do two things. First, one must describe the properties that the set – theoretic versions of these number systems should satisfy. Second, something should be said to justify our describing such systems as THE natural numbers, THE integers, THE rational numbers, and THE real numbers. This usage suggests that we have completely unambiguous descriptions of the number systems in terms of their algebraic and other properties. One way of stating this is that any system satisfying all the conditions for one of the standard systems N, Z, Q or R should be the same as N, Z, Q or R for all mathematical purposes, with some explicit means for mechanical translation from the given system to the appropriate standard model.
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