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Re:Set Theory and Foundations of Mathematics by dmead on Friday July 31, 2015 @10:14AM (#9981847)
i blew a google interview question because i failed to recognize a powerset. keeping up with this stuff is pretty important.
Re:Set Theory and Foundations of Mathematics by rectang on Friday July 31, 2015 @11:37AM (#9982387)
It's not really your fault, but defeating Google-gotcha interview questions is a very distorting motivation. I'd like to think that this material is worth studying because it will help you to achieve things you otherwise could not, whether at Google or elsewhere.

But that leads to a different kind of study: deep dives into specialized subjects relevant for the task at hand, rather than attempting to maintain shallow coverage over a broad field.

Re:Set Theory and Foundations of Mathematics by acrefoot on Friday July 31, 2015 @12:33PM (#9982697)
If they specifically asked for "powerset" that might be considered a gotcha question, but a powerset is another way of thinking of binary enumeration[0], which isn't that rare.

[0] by that I mean both the case of counting in binary, as well as enumerating all the ways you can have or not have some things--you might care about that when considering all the interactions of config flags you might have to deal with.

Re:Set Theory and Foundations of Mathematics by dmead on Friday July 31, 2015 @12:37PM (#9982715)
this is exactly the case. had i just remember what a power set was instead of say something dumb like "all the n-k combination of these children something someting" it would have gone much better.

and yes, it was a hard tree question.

Re:Set Theory and Foundations of Mathematics by gooseus on Friday July 31, 2015 @10:20AM (#9981884)
I tried to find it in the OP, it sounded way over my head... this link explained it real fast.

https://www.mathsisfun.com/sets/power-set.html

I still wish I could find the time to study all this foundational math... super interesting and I have a sneaky suspicion the answer to a great many philosophical questions are hidden in there.

Re:Set Theory and Foundations of Mathematics by vowelless on Friday July 31, 2015 @10:55AM (#9982124)
> I have a sneaky suspicion the answer to a great many philosophical questions are hidden in there.

Godel's incompleteness theorem had a profound affect on me that I find hard to quantify. I grew up starry eyed, thinking that our minds have virtually limitless capabilities and that mathematics could answer everything (naive, I know). Then I came across Russell's paradox and the incompleteness theorems and for the first time (late high school) got confronted by the limits of our 'tools' (with proof!). And then encountering Heisenberg's uncertainty principle was just depressing and I took to computer science.

I think better than just looking up seemingly random mathematical concepts is to go through the history of the development of set theory.

Re:Set Theory and Foundations of Mathematics by sbilstein on Friday July 31, 2015 @12:24PM (#9982642)
I just finished reading Godel's Proof (a non-textbook account of the proof recommended by Hofstader in GEB) and it was mind-blowing. I really recommend that and David Foster Wallace's Beyond Infinity. Beyond Infinity is great history of set theory and written in an exciting and fun style.

I came across the ideas the opposite way...I loved math as child and as engineer but I always felt like systems of rationalization had to be limited in some way. I think I started feeling this way when I'd get into debates with other students in school about arguments and we'd have equally rational but contradictory viewpoints.

Fast forward a few years later and I learn some genius had actually proved that we can't answer everything with these systems...or that there isn't a system that can say it all.

Re:Set Theory and Foundations of Mathematics by EvaK_de on Friday July 31, 2015 @10:23PM (#9985609)
The book by DFW sounds interesting, but all I could find is "Everything and More: A Compact History of Infinity". Is that it?
Re:Set Theory and Foundations of Mathematics by sbilstein on Sunday August 02, 2015 @02:17PM (#9991692)
Sorry yea, I misremembered the title when I was commenting.
Re:Set Theory and Foundations of Mathematics by valera_rozuvan on Friday July 31, 2015 @01:25PM (#9982976)
> Fast forward a few years later and I learn some genius had actually proved that we can't answer everything with these systems...or that there isn't a system that can say it all.

So true...

Re:Set Theory and Foundations of Mathematics by pjungwir on Friday July 31, 2015 @01:47PM (#9983141)
I felt similar amazement at Gödel's proof, and even suspect that Penrose must be on to something when he says the mind must therefore not be computable. . . . But the book Gödel's Theorem: An Incomplete Guide to its Use and Abuse by Törkel Franzén is very good at explaining that the conclusions we should draw are fairly limited, e.g. that the proof doesn't apply to anything other than arithmetic. I'm not sure I'm totally convinced, but it's a great perspective to dispel some of the more mystical interpretations.
Re:Set Theory and Foundations of Mathematics by GeneTT on Friday July 31, 2015 @08:30PM (#9985306)
Dennet also has a pretty good take down of Penrose in chapter 15 of Darwins Dangerous Idea.
Re:Set Theory and Foundations of Mathematics by valera_rozuvan on Friday July 31, 2015 @11:49AM (#9982454)
Best article I read on the history of development of set theory is by José Ferreirós: "The Early Development of Set Theory". It can be found at http://plato.stanford.edu/entries/settheory-early/ .
Re:Set Theory and Foundations of Mathematics by Sirenos on Friday July 31, 2015 @11:15AM (#9982261)
Isn't Russell's paradox more like a problem with naive set theory? By analogy to a software framework, the paradox would be considered a bug in the framework and say nothing about how useful it is.
Re:Set Theory and Foundations of Mathematics by tikhonj on Friday July 31, 2015 @01:52PM (#9983171)
In a sense. But it's interesting because naive set theory appears so simple and natural: it's a concrete example of how human ideas that seem platonic and inviolate can actually be inconsistent.

So it's less like finding a bug in a software framework and more like finding a bug in your understanding of how computers can work.

Re:Set Theory and Foundations of Mathematics by OJFord on Friday July 31, 2015 @03:40PM (#9983895)
> I grew up starry eyed, thinking that our minds have virtually limitless capabilities ... and for the first time (late high school) got confronted by the limits of our 'tools' ... was just depressing and I took to computer science.

Alas, not even safe there. Thanks a lot, Turing.

Re:Set Theory and Foundations of Mathematics by Shaniqua on Friday July 31, 2015 @10:59AM (#9982157)
But Russel's Paradox is easy to fix.

Let x be a set. Then, V = {x| x not in x} is the set that causes Russel's Paradox. We can easily define a new set S = {x| p(x) and x in U} where p(x) is some property and U is the universal set. Then S easily fixes the contradiction.

Re:Set Theory and Foundations of Mathematics by dvt on Friday July 31, 2015 @01:48PM (#9983147)
Having a universal set [in naive set theory] is a sufficient condition for Russel's paradox. See Naive Set Theory[1] bottom of page 6. This is why we can have no Universe in any consistent set theory.

Edit: @mafribe makes the point that there are some set theories that can still have universal sets by culling other features that ZF-style set theories have. I was mostly referring to ZF-style set theory (hence my citation). Indeed, one could even make a ZF-style set theory paraconsistent and still have Universal sets.

[1] http://sistemas.fciencias.unam.mx/~lokylog/images/stories/Al...

Re:Set Theory and Foundations of Mathematics by mafribe on Friday July 31, 2015 @02:22PM (#9983384)

   Having a universal set is a sufficient condition for Russel's paradox.
That's not true. There are set-theories, e.g. Quine's NF [1] which allow universal sets, and other things like the set of all ordinals, that are forbidden in ZF-style set-theories. The problem in ZF is caused by unlimited comprehension. NF circumvents this by restricting comprehension. Tom Forster [2] has written a great deal about set theories with universal sets, including the wonderful [3]. He makes the historical point that set theory was born with universal sets.

[1] http://plato.stanford.edu/entries/quine-nf/

[2] https://www.dpmms.cam.ac.uk/~tf/

[3] T. E. Forster, Set Theory with a Universal Set. http://ukcatalogue.oup.com/product/9780198514770.do

Re:Set Theory and Foundations of Mathematics by dvt on Friday July 31, 2015 @02:34PM (#9983459)
True, I was mostly referring to ZF-style set theories (which is what the thread is mainly about). Your point could even be extended by saying that there are proofs for a paraconsistent ZF with a universal set.

Your [3] link doesn't work by the way, I'm interested in reading Forster!

Re:Set Theory and Foundations of Mathematics by mafribe on Saturday August 01, 2015 @03:32AM (#9986271)
[3] works on my browser. Anyway, the link was to the publisher's page for the book. Here is another one: http://www.amazon.co.uk/Set-Theory-Universal-Exploring-Unive... .
Re:Set Theory and Foundations of Mathematics by Shaniqua on Friday July 31, 2015 @02:42PM (#9983509)
You can fix the Russel's Paradox in ZF as well.
Re:Set Theory and Foundations of Mathematics by banachtarski on Friday July 31, 2015 @11:25AM (#9982323)
I have no idea where you got this from but your statements don't follow any form of logic I'm familiar with.

If V = {x | x not in x}, then if V contains itself, V is not in V (and vice versa) is an obvious contradiction. Your new set S doesn't help in the slightest.

Fixing it is emphatically "not easy" and mathematicians generally rely on the ZFC axiomation (although several other possibilities were proposed).

Re:Set Theory and Foundations of Mathematics by Shaniqua on Friday July 31, 2015 @11:52AM (#9982465)
It's no big deal if you don't admit universal set or anything other than ZFC.
Re:Set Theory and Foundations of Mathematics by johntb86 on Friday July 31, 2015 @03:05PM (#9983663)
It took 16 years (1901-1917) to get from russell's paradox to a set theory which didn't allow it, but which was able to create a lot of interesting and useful sets (ZF). So it seems like a big deal. And we still can't talk about "the set/collection/whatever of all sets" in the language of ZFC, so we're still missing out.
Re:Set Theory and Foundations of Mathematics by defen on Friday July 31, 2015 @12:17PM (#9982601)
> universal set

But now you're talking about non-standard set theory[1] which is fine but you are kind of side-stepping the issue.

[1] https://en.wikipedia.org/wiki/Axiom_of_regularity

Re:Set Theory and Foundations of Mathematics by Shaniqua on Friday July 31, 2015 @12:38PM (#9982721)
Well, it's not that difficult to fix it in ZF.

If you define S to be S = {x in U: x not in x}, then it simply means S is not in U.

Re:Set Theory and Foundations of Mathematics by defen on Friday July 31, 2015 @01:32PM (#9983032)
There is no U in ZF so I really don't understand what you are saying.
Re:Set Theory and Foundations of Mathematics by Shaniqua on Friday July 31, 2015 @01:57PM (#9983218)
You can call U anything you want. It's by axiom schema of specification: for any set A, there some set B with a set C in B iff C in A.
Re:Set Theory and Foundations of Mathematics by voidz on Friday July 31, 2015 @11:14AM (#9982256)
Why is this downvoted? Is the statement incorrect?
Re:Set Theory and Foundations of Mathematics by banachtarski on Friday July 31, 2015 @11:25AM (#9982327)
Yes
Re:Set Theory and Foundations of Mathematics by Shaniqua on Friday July 31, 2015 @01:14PM (#9982926)
By the way, I forgot to ask do you object to my post about "killing" the Russel set because you don't accept the existence of universal set or because you don't understand how {x in S| x not x} helps here? If it's the former, can you assume that U exists and explain how {x in S| x not x} solves the problem? So that I know you're not simply "saying things".
Re:Set Theory and Foundations of Mathematics by voidz on Friday July 31, 2015 @12:00PM (#9982510)
Ah, I see you explained. Thanks. I love the subject, but I'm still a neophyte.
Re:Set Theory and Foundations of Mathematics by SchemeLisp on Friday July 31, 2015 @10:54AM (#9982115)
Back when I just started learning all this stuff, I used to confuse elements of a set and its subsets. For example, I'd be easily stumped when asked what the elements and subsets of {a} were. Also, it's good to know why the empty set is a subset of any set and the proof/s that there are 2^n elements in a power set.
Re:Set Theory and Foundations of Mathematics by tel on Friday July 31, 2015 @11:55AM (#9982487)
> I have a sneaky suspicion...

You have no idea. The more I study this stuff the more I feel that all philosophical questions might be somehow encoded in there.

Re:Set Theory and Foundations of Mathematics by AnimalMuppet on Friday July 31, 2015 @12:23PM (#9982639)
Sartre said that the great philosophical question is, why is there something instead of nothing? (This does not mean, why is there something in philosophy. Why is there something in the real world? Why does anything physically exist?) If your philosophy doesn't explain the real world, it isn't much of a philosophy. But I don't think set theory can explain that.

Two other great philosophical questions are: Where do we humans find meaning? And, what is the basis for morals and ethics? How do we determine what actions are right and wrong? Set theory isn't going to answer those questions at all.

Re:Set Theory and Foundations of Mathematics by anthony_d on Friday July 31, 2015 @01:24PM (#9982969)
Sorry, but "Where do we humans find meaning?" isn't even a mediocre philosophical question. It's navel gazing by angst ridden teenagers.
Re:Set Theory and Foundations of Mathematics by tel on Friday July 31, 2015 @02:00PM (#9983237)
It was a bit tongue-in-cheek, but I'd argue that mathematical foundations (maybe not set theory interpreted strictly) does take aim at answers to "why is there something?" and "where do humans find meaning?". Possibly by unifying the questions!
Re:Set Theory and Foundations of Mathematics by valera_rozuvan on Friday July 31, 2015 @01:28PM (#9982999)
> Two other great philosophical questions are: Where do we humans find meaning? And, what is the basis for morals and ethics?

I think the best question is "What will humans do, when we find meaning to everything?". I hope that never happens.

Re:Set Theory and Foundations of Mathematics by mindcrime on Friday July 31, 2015 @12:41PM (#9982740)
Two other great philosophical questions are: Where do we humans find meaning? And, what is the basis for morals and ethics? How do we determine what actions are right and wrong?

And Camus said that the one truly interesting philosophical problem is suicide.

http://www.camus-society.com/myth-of-sisyphus.html

Set theory isn't going to answer those questions at all.

Indeed.

Re:Set Theory and Foundations of Mathematics by vezzy-fnord on Friday July 31, 2015 @01:14PM (#9982927)
Only if you're biased towards a realist philosophical view. Though admittedly it makes many feel warm and fuzzy, there's really no evidence for it over positions like intuitionism.
Re:Set Theory and Foundations of Mathematics by tel on Friday July 31, 2015 @01:49PM (#9983155)
I'm not, but I can't see how you cannot study set theory without eventually studying intuitionism.
Re:Set Theory and Foundations of Mathematics by rwallace on Friday July 31, 2015 @04:25PM (#9984225)
Well, that's like saying you blew a Google interview question because you failed to recognize a black vine weevil, therefore keeping up with entomology is pretty important. One company having a buggy interview process that asks questions irrelevant to the job, at the very most means if you are applying for a job at that company you should spend a couple days cramming for the interview. It does not mean you should spend the precious hours of your life studying a field that's irrelevant to your job on an ongoing basis.

To be clear, if you want to study set theory or entomology because you find them interesting for their own sake then far be it from me to criticize that decision. I'm just disagreeing with the argument that you should do so because of one company's buggy interview process.