Discussion

Not all elementary functions can be expressed with exp-minus-log

lotaezenwa: The author essentially says that the quintic has no closed form solution which is true regardless of the exp-minus-log function. The purpose of this blog post is lost on me.Can anyone please explain this further? It seems like he’s moving the goalposts.

markgall: Can anyone provide a link that "Some are going as far as to suggest that the entire foundations of computer engineering and machine learning should be re-built as a result of this", or anything similarly grandiose?I am a professional mathematician, though nowhere near this kind of thing. The result seems amusing enough, but it doesn't really strike me as something that would be surprising. I confess that this thread is the first I've heard of it...

avmich: I'd really like more details on the terminology used.Also I'd be glad to see a specific example of a function, considered elementary, which is not representable by EML.It could be hard, and in any case, thanks for the article. I wish it would be more accessible to me.

AlotOfReading: The argument is that a universal basis would be capable of solving arbitrary polynomial roots. The rest is an argument that the group constructed by eml is solveable, and hence not all the standard elementary functions.It wouldn't be a math discussion without people using at least two wildly different definitions.

saithound: The original article explicitly acknowledged this limitation, that while in "the classical differential-algebraic setting, one often works with a broader notion of elementary function, defined relative to a chosen field of constants and allowing algebraic adjunctions, i.e., adjoining roots of polynomial equations," the author works with the less general definition.Neither the present article, nor the original one has much mathematical originality, though: Odrzywolek's result is immediately obvious, while this blog post is a rehash of Arnold's proof of the unsolvability of the quintic.

petters: Yes, that blog post could have been much shorter….

DevelopingElk: His claim is that we exp-minus-log cannot compute the root of an arbitrary quintic. If you consider the root of an arbitrary quintic "elementary" the exp-minus-log can't represent all elementary functions.I think it really comes down to what set of functions you are calling "elementary".

throwanem: The author discusses this in his third paragraph, and states explicitly he considers the result faulty for its unrealistically narrow definition of elementarity.

markgall: I only skimmed the article, but I think the idea is to use some variation on:f(a,b,c,d,e) = the largest real solution x of the quintic equation x^5 + ax^4 + bx^3 + cx^2 + dx + e = 0There's not a simple formula for this function (which is the basic point), but certainly it is a function: you feed it five real numbers as input, and it spits out one number as output. The proof that you can't generate this function using the single one given looks like some fairly routine Galois theory.Whether this function is "considered elementary" depends on who you ask. Most people would not say this is elementary, but the author would like to redefine the term to include it, which would make the theorem not true anymore.Why any of this would shake the foundations of computer engineering I do not know.

SabrinaJewson: Related is the paper [What is a closed-form number?], which explores the field E, defined as the smallest subfield of ℂ closed under exp and log. I believe the set of numbers that can be generated using exp-minus-log is a strict subset of this.In a similar vein to this post, the paper points out that general polynomials do not have solutions in E, so of course exp-minus-log is similarly incomplete.What is intriguing is that we don’t even know whether many simple equations like exp(-x) = x (i.e. the [omega constant]) have solutions in E. We of course suspect they don’t, but this conjecture is not proven: https://en.wikipedia.org/wiki/Schanuel%27s_conjectureWhat is a closed-form number?: http://timothychow.net/closedform.pdf omega constant: https://en.wikipedia.org/wiki/Omega_constant

reikonomusha: "The quintic has no closed form solution" is a theorem that is more precisely stated as follows: The quintic has no closed form solution in terms of arbitrary compositions of rational numbers, arithmetic, and Nth roots. We can absolutely express closed form solutions to the quintic if we broaden our repertoire of functions, such as the Bring radical. The post's argument different than the usual Galois theory result about the unsolvability of the quintic, in that it shows that even with a transcendental function like EML(x, y), we still don't get a solution.

saithound: It's an unsurprising undergrad-level result. It got picked up and overhyped on HN [1] and /r/math [2] earlier this week.Some of my favorites:DoctorOetker: "I'm still reading this, but if this checks out, this is one of the most significant discoveries in years."cryptonektor: "Given this amazing work, an efficient EML operator HW implementation could revolutionize a bunch of things."zephen: "This is about continuous math, not ones and zeroes. Assuming peer review proves it out, this is outstanding."[1] https://news.ycombinator.com/item?id=47746610[2] https://www.reddit.com/r/math/comments/1sk63n5/all_elementar...

bawolff: > Elementary functions typically include arbitrary polynomial rootsAdmittedly this may be above my math level, but this just seems like a bad definition of elementary functions, given the context.

Aardwolf: But can you even express this function with the elementary operator symbols, exp, log, power and trig functions? It seems to me like no, you can't express the "largest real solution" aspect with those

avmich: I've thought something like that, but I'm interested more in details of the argument.As for why this could be important... we sometimes find new ways of solving old problems, when we formulate them in a different language. I remember how i was surprised to learn how representation of numbers as a tuple (ordered list of numbers), where each element is the remainder for mutually prime dividers - as many dividers as there are elements in the tuple - reduces the size of tables of division operation, and so the hardware which does the operation using thise tables may use significantly less memory. Here we might have some other interesting advantages.

renewiltord: This result itself is being described in those terms[1]:> If this is true, then this blog post is going to up-end all of mathematics for the next century.This is very concerning for mathematics in general.1: https://news.ycombinator.com/item?id=47775105

renewiltord: If this is true, then this blog post is going to up-end all of mathematics for the next century.

bawolff: Well the author saysin that paragraph:> In layman’s terms, I do not consider the “Exp-Minus-Log” function to be the continuous analog of the Boolean NAND gate or the universal quantum CCNOT/CSWAP gates.But is there actually a combination of NANDs that find the roots of an arbitrary quintic? I always thought the answer was no but admittedly this is above my math level.

zarzavat: This is a bit like invalidating a result based on 0^0 := 1 because you work in a field of mathematics where 0^0 is an indeterminate form. Not very interesting.AFAIU the original paper is a result in the field of symbolic regression. What definition of elementary function do they use?

rnhmjoj: > My concern is that the word “elementary” in the title carries a much broader meaning in standard mathematical usage, and in this meaning, the paper’s title does not hold.> Elementary functions typically include arbitrary polynomial roots, and EML terms cannot express them.If you take a real analysis class, the elementary functions will be defined exactly as the author of the EML paper does.I've actually just learnt that some consider roots of arbitrary polynomials being part of the elementary functions before, but I'm a physicist and only ever took some undergraduate mathematics classes. Nonetheless, calling these elementary feels a bit of stretch considering that the word literally means basic stuff, something that a beginner will learn first.

reikonomusha: Combinations of the NAND gate can express any Boolean function. The Toffoli (CCNOT) or Fredkin (CSWAP) can express any reversible Boolean function, which is important in quantum computing where all gates must be unitary (and therefore reversible). The posited analog is that EML would be the "universal operator" for continuous functions.

teo_zero: I feel that saying that EML can't generate all the elementary functions because it can't express the solution of the quintic is like saying that NAND gates can't be the basis of modern computing because they can't be used to solve Turing's halting problem.

vintermann: Yes, this article is kicking in open doors, the original article was quite clear about the scope.The present article could rather have spent time arguing why this isn't like NAND gate functional completeness.I would have thought the differences lie in the other direction: not that trees of EML and 1 can describe too little, but that they can describe too much already. It's decidable whether two NAND circuits implement the same function, I'm pretty sure it's not decidable if two EML trees describe the same function.

reikonomusha: The definition of "elementary function" typically includes functions which solve polynomials, like the Bring radical. The definition is most fitting in algebraic contexts where algebraic structure is meaningful (like Liouvillian structure theorems). See e.g.- Page 2 and the following example of https://billcookmath.com/courses/math4010-spring2016/math401... (2016)- Ritt's Integration in Finite Terms: Liouville's Theory of Elementary Methods (1948)It's not frequent that analysis books will define the class of elementary functions rigorously.