Hunsecker
The two proofs in the OP were of interest because they seemed to prove contradictory hypotheses. Proof 1, with merely the slightest variation, specifically leaving out the definition that .9… = .9[n], has been for a long, long time a famous proof that .9… = 1. One implication of this equality is that the set of real numbers is an infinite set. If .9… did not equal 1, then the implication is that there exist two real numbers not equal, and with nothing in between them.
The same point can be made without the definition though, by asking, however naively, where the extra 9 comes from when we multiply .9… by 10 to get 9.9… In other words, 10 times 9.999 equals 99.99, and not 99.999.
Those here who held up Cantor (whose genius I certainly recognize) were not mindful of the paradox raised in his Mengenlehre (Epistemology of Sets), which Russell did recognize, and which he solved by his theory of type sets. Great minds, Zermelo, Fraenkel, Bernays, and von Neumann all saw the problem with Cantor’s transfinite ordinals in that he believed that the infinte aggregate of all ordinals, having stated that every set can be well ordered, is itself well ordered.
Russell illustrated the paradox this way: In a small village, the barber shaves all those men and only those men who do not shave themselves. Now, the question is, who shaves the barber? If he is one of those men who do not shave themselves, then he must be shaved by the barber. But he is the barber. So if he shaves himself, then he does not shave himself. Oops.
Russell introduced the theory of types, such that individual elements are of type 0, classes of individuals are type 1, classes of classes are type 2, and so on. Thus, individuals can be elements of a class, and a class can be an element of a class of classes, and so on. In general, anything of type n can be an element of a class of type n + 1, but a class cannot be a member of itself. That means that the barber paradox is a confusion of types of classes, and thus the paradox is resolved.
And that is the ultimate reason that .9[n] fails as a definition for .9…, namely that .9… is an element of a class of one type (a number whose digits represent a transfinite cardinality), wherease .9[n] is an element of a different kind of class (numbers whose digits are finite and countable).
I had originally developed Proof 2 as an inductive proof, but ran across a lot of understandable resistance on the basis of a deductive proof (like Proof 1) being “superior” to an inductive proof. So, I simply made what I had induced before into an axiom, which allowed me to develop the proof deductively. And it is a very useful tool for certain things. It is true that 4 times .7[n] equals 3.1[n-2]08, but only when n is finite.
Father Mentock was the one who zeroed right in on the problem of .9[n] times 10 not being .9[n-1]. He then proceeded, in a very gentlemanly way, to show how transfinite ordinalities are derived, not from counting sets, but from operations on counting sets that have transfinite solutions. In fact, it is common that operations on real numbers, where real numbers are the universal set, often produce solutions that are empty in the universal set. One example is x^2 = -1. If you solve for x, you will need to expand your universal set to be the set of complex numbers if you are not satisfied with {} as your answer.
Math is utterly replete with the introduction of new fields that are formed all the time by simple rearrangement of other fields: dropping an axiom here, tweaking a definition there. Noneuclidean geometry arose in exactly this way, by simply dropping the parallel line postulate.
But despite what some have said in this thread, an awful lot remains to be done in number theory with respect to both cardinality and ordinality. Infinity, interestingly enough, is not the final word. Nor has infinity been “pinned down”. There is nothing wrong with questioning Russell now, for example, and seeking to formulate a more cogent resolution to the paradox of infinite ordinals.
If everybody just stops and says, “oh well, it’s all finished,” then we won’t ever learn anything new.
“It is lucky for rulers that men do not think.” — Adolf Hitler