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| ▲ | empath75 4 days ago | parent | next [-] | | He doesn't work with imaginary numbers, either. He treats complex numbers as matrices of rationals. | | |
| ▲ | numpy-thagoras 4 days ago | parent [-] | | Which is the same thing for all intents and purposes. An ultrafinitist is still allowed to call that 'i'. | | |
| ▲ | dhosek 4 days ago | parent [-] | | Still kind of freaked out that a Möbius transform can be expressed as a matrix multiplication. |
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| ▲ | vlovich123 4 days ago | parent | prev | next [-] | | How do you rationalize pi or e? | | |
| ▲ | empath75 4 days ago | parent | next [-] | | You don't. He basically defines numbers like pi and e not as numbers, but as iterative functions, which you can run to whatever level of accuracy that you want. It's sort of a silly argument, because _all_ numbers can be treated like the output of a function, including the real numbers, so he has basically smuggled in all reals through the back door, because any real number can just be thought of as a function with increasingly precise return values with an infinitely long description, just like pi is. | | |
| ▲ | MarkusQ 4 days ago | parent [-] | | You can't get all the reals that way. The reals that can be produced by an algorithm make up a vanishingly small (e.g. countable) subset. Almost all of the reals are inexpressible. | | |
| ▲ | empath75 4 days ago | parent | next [-] | | What I described isn't really an algorithm, it's just taking the digits of a number, let's say: foo=3.14159265... Where after 5 is some continuing sequence of decimals. The series of functions is literally just: foo(0) = 3
foo(1) = 3.1
foo(2) = 3.14... And to be clear, it's not just like, an algorithm that estimates pi, it's literally just a list of return values that is infinitely long that return more and more digits of whatever the number is. That is actually how he defines pi. https://youtu.be/lcIbCZR0HbU?si=3YxcHfPlCFrlr5h3&t=2080 pi _happens_ to be computable, and there are more efficient functions that will produce those numbers, but you could do the same thing with an incomputable number, you just need a definition for the number which is infinitely long. To be clear, I don't think any of this is a good idea, just pointing out that if he's going to allow that kind of definition of pi (ie, admit a definition that is just an infinite list of decimal representations), you can just do the same thing with any real number you like. He of course will say that he's _not_ allowing any _infinite list_, only an arbitrary long one. | | |
| ▲ | MarkusQ 2 days ago | parent [-] | | That's the key point though, this list isn't infinitely long, and all the numbers in it are rational. And it is an algorithm (specifically, a lookup table). All the numbers you get this way are going to be rational, and if you require them to be finite, you can't even identify them with any irrational numbers. At least with the computable numbers you get an infinite set of irrational numbers along with the rationals, while still never touching the vast majority of all numbers (the remaining, incomputable irrationals). |
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| ▲ | LPisGood 4 days ago | parent | prev [-] | | To an ultrafinitist, there is no such thing as a number that is inexpressible. | | |
| ▲ | kragen 4 days ago | parent [-] | | Right, but to be clear, it's not that ultrafinitists like Wildberger believe that they can express all the real numbers; rather, they believe that those inexpressible real numbers don't actually exist. | | |
| ▲ | vlovich123 4 days ago | parent [-] | | How does that work for calculus which regularly looks at the limits of functions as x approaches infinity and has very real real world applications that stem from such algorithms? | | |
| ▲ | LPisGood 3 days ago | parent | next [-] | | Here is a paper on just how a serious ultrafinitist copes with that https://sites.math.rutgers.edu/~zeilberg/mamarim/mamarimPDF/... The short answer is that they deal with such things symbolically. | |
| ▲ | drdec 4 days ago | parent | prev [-] | | Math in general needs to have a big blinking "don't confuse the map for the territory" label on it. E.g. when you calculate the area of a plot of land do you take into account the curvature of the Earth? You have to make a bunch of compromises in the first place to even talk about what the area of a plot land means. Math is a bunch of useful systems that we humans have devised. We tend to gravitate towards the ones that help us describe and predict things in the real world. But there is plenty of math which doesn't do either. It's just as real as the math that does. | | |
| ▲ | vlovich123 3 days ago | parent [-] | | I agree. I’m just trying to speak to the “realist” argument that Wilderberg presents claiming that some numbers aren’t real and there’s no point talking about them when they come from very practical mathematics let alone the ones that aren’t. In no way was I trying to claim that some part of maths aren’t real - I was trying to understand the consistency of what to me seems like a confusing argument to make. | | |
| ▲ | kragen 3 days ago | parent [-] | | I don't think uncomputable numbers "come from very practical mathematics"! Rather, they come from Gödel, Church, and Turing demolishing Hilbert's program of solving the Entscheidungsproblem once and for all. Possibly, if Hilbert had succeeded, that would have made it "very practical mathematics", or possibly not, but that counterfactual is reasoning from a logical contradiction. https://plato.stanford.edu/entries/church-turing/decision-pr... |
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| ▲ | aarestad 4 days ago | parent | prev [-] | | By fiat, of course. :) (e.g. https://en.wikipedia.org/wiki/Indiana_pi_bill) |
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| ▲ | 7thaccount 4 days ago | parent | prev [-] | | I never said they were, but could've sworn that the Wikipedia page or parent comment did (I can't find it now and am questioning my sanity). I couldn't understand how he could try to get rid of them, although this isn't surprising as mathematics is basically magic to me once you get past calculus. I guess this is only about removing irrationals though. | | |
| ▲ | griffzhowl 4 days ago | parent [-] | | It depends on the particular construction. You could construct the "complex rational field" by adding i to the rationals with the rule i^2 = -1. That seems to be what Wildberger is ok with. The standard complex numbers involve adding i to the real numbers, which Wildberger doesn't like. I don't know how you'd do electrical engineering with the rational complex field, because electrical engineering and physics in general involves a lot of irrational quantities and calculus, and the standard foundations of these concepts use real numbers. It's really up to finitists to show that there are problems with these methods and that they have a better way of doing things, because so far the standard way seems to work very well. |
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