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Comment author: BrienneYudkowsky 24 September 2013 04:39:17AM 3 points [-]

It's grue. Clearly.

Comment author: David_Kristoffersson 26 October 2016 08:57:49AM *  0 points [-]

It's bleen, without a moment's doubt.

In response to LessWrong 2.0
Comment author: gwillen 03 December 2015 08:17:58AM 19 points [-]

I would caution away from a bias towards "the current situation seems vaguely bad, therefore Something Must Be Done." There are lots of people still getting use out of LessWrong. I think it would be unfortunate that a bias towards Doing Something over Leaving It Be might cause a valuable resource to be ended without good cause. If the site can be reinvented, great, but if it can't -- don't hit the Big Red Button without honestly weighing the significant costs to the people who are still actively using the site.

(I briefly searched, to see if there's an article on LW about the idea of a bias towards Doing Something. It would of course be essentially the opposite of status quo bias; and yet I think it's a real phenomenon. I certainly feel like I observe it happening in discussions like this. Perhaps the real issue is in the resolution of conflicts between a small minority who are outspoken about Doing Something, and a large silent majority who don't express strong feelings because they're fine with the status quo. This is an attempt to express a thought that I've had percolating, not a criticism of this post.)

In response to comment by gwillen on LessWrong 2.0
Comment author: David_Kristoffersson 08 May 2016 10:06:56AM 0 points [-]

Counterpoint: Sometimes, not moving means moving, because everyone else is moving away from you. Movement -- change -- is relative. And on the Internet, change is rapid.

Comment author: David_Kristoffersson 09 October 2015 07:11:29PM 0 points [-]

Interesting. I might show up.

Comment author: Vladimir_Nesov 15 August 2015 01:16:29PM 6 points [-]

When I first worked through this book, it didn't result in long-term retention of the material (I'm sure some people will be able to manage, just not me, not without meditating on it much longer than it takes to work through or setting up a spaced repetition system). In that respect, Enderton's Elements of Set Theory worked much better. Enderton's book goes into more detail, giving enough time to exercise intuition about standard proofs. At the same time, it's an easier read, which might be helpful if Halmos's text seems difficult.

Comment author: David_Kristoffersson 16 August 2015 10:26:47AM *  2 points [-]

Thanks for the tip. Two other books on the subject that seem to be appreciated are Introduction to Set Theory by Karel Hrbacek and Classic Set Theory: For Guided Independent Study by Derek Goldrei.

Edit: math.se weighs in: http://math.stackexchange.com/a/264277/255573

Comment author: Tyrrell_McAllister 15 August 2015 03:15:45PM 4 points [-]

In general, reading about the same subject from a different author is a great way to learn and retain the material better. This is true even if neither author is objectively "better" than the other. Something about recognizing the same underlying concept expressed in different words helps to fix that concept in the mind.

It's possible to exploit this phenomenon even when you have only one text to work with. One trick I use when working through a math text is to willfully use different notation in my notes next to the text. Using a different notation forces me to make sure that I'm really following the details of the argument. Expressing the same logic in different symbols makes it easier to see through those symbols to the underlying logic.

Comment author: David_Kristoffersson 16 August 2015 10:23:09AM *  2 points [-]

The author of the Teach Yourself Logic study guide agrees with you about reading multiple sources:

I very strongly recommend tackling an area of logic (or indeed any new area of mathematics) by reading a series of books which overlap in level (with the next one covering some of the same ground and then pushing on from the previous one), rather than trying to proceed by big leaps.

In fact, I probably can’t stress this advice too much, which is why I am highlighting it here. For this approach will really help to reinforce and deepen understanding as you re-encounter the same material from different angles, with different emphases.

Comment author: stoat 15 August 2015 03:39:10PM 2 points [-]

Looks to me like Halmos does intend "one-to-one" to mean "injective". What he writes is "A function that always maps distinct elements onto distinct elements is called one-to-one (usually a one-to-one correspondence)." Then he mentions inclusion maps as examples of one-to-one functions.

Comment author: David_Kristoffersson 16 August 2015 09:59:22AM *  0 points [-]

My two main sources of confusion in that sentence are:

  1. He says "distinct elements onto distinct elements", which suggests both injection and surjection.
  2. He says "is called one-to-one (usually a one-to-one correspondence)", which might suggest that "one-to-one" and "one-to-one correspondence" are synonyms -- since that is what he usually uses the parantheses for when naming concepts.

I find Halmos somewhat contradictory here.

But I'm convinced you're right. I've edited the post. Thanks.

Comment author: gjm 15 August 2015 03:55:11PM 0 points [-]

Oh yes, for sure, but the context here was a statement that "onto" means surjective while "one-to-one" means bijective. Definitely talking functions. And I would be really surprised if Halmos were using "one-to-one" followed by anything other than "correspondence" to mean bijective.

Comment author: David_Kristoffersson 16 August 2015 09:53:22AM 0 points [-]

You guys must be right. And wikipedia corroborates. I'll edit the post. Thanks.

Book Review: Naive Set Theory (MIRI research guide)

13 David_Kristoffersson 14 August 2015 10:08PM

I'm David. I'm reading through the books in the MIRI research guide and will write a review for each as I finish them. By way of inspiration from how Nate did it.

Naive Set Theory

Halmos Naive Set Theory is a classic and dense little book on axiomatic set theory, from a "naive" perspective.

Which is to say, the book won't dig to the depths of formality or philosophy, it focuses on getting you productive with set theory. The point is to give someone who wants to dig into advanced mathematics a foundation in set theory, as set theory is a fundamental tool used in a lot of mathematics.


Is it a good book? Yes.

Would I recommend it as a starting point, if you would like to learn set theory? No. The book has a terse presentation which makes it tough to digest if you aren't already familiar with propositional logic, perhaps set theory to some extent already and a bit of advanced mathematics in general. There are plenty of other books that can get you started there.

If you do have a somewhat fitting background, I think this should be a very competent pick to deepen your understanding of set theory. The author shows you the nuts and bolts of set theory and doesn't waste any time doing it.

Perspective of this review

I will first refer you to Nate's review, which I found to be a lucid take on it. I don't want to be redundant and repeat the good points made there, so I want to focus this review on the perspective of someone with a bit weaker background in math, and try to give some help to prospective readers with parts I found tricky in the book.

What is my perspective? While I've always had a knack for math, I only read about 2 months of mathematics at introductory university level, and not including discrete mathematics. I do have a thorough background in software development.

Set theory has eluded me. I've only picked up fragments. It's seemed very fundamental but school never gave me a good opportunity to learn it. I've wanted to understand it, which made it a joy to add Naive Set Theory to the top of my reading list.

How I read Naive Set Theory

Starting on Naive Set Theory, I quickly realized I wanted more meat to the explanations. What is this concept used for? How does it fit in to the larger subject of mathematics? What the heck is the author expressing here?

I supplemented heavily with wikipedia, math.stackexchange and other websites. Sometimes, I read other sources even before reading the chapter in the book. At two points, I laid down the book in order to finish two other books. The first was Gödel's Proof, which handed me some friendly examples of propositional logic. I had started reading it on the side when I realized it was contextually useful. The second was Concepts of Modern Mathematics, which gave me much of the larger mathematical context that Naive Set Theory didn't.

Consequently, while reading Naive Set Theory, I spent at least as much time reading other sources!

A bit into the book, I started struggling with the exercises. It simply felt like I hadn't been given all the tools to attempt the task. So, I concluded I needed a better introduction to mathematical proofs, ordered some books on the subject, and postponed investing into the exercises in Naive Set Theory until I had gotten that introduction.


In general, if the book doesn't offer you enough explanation on a subject, search the Internet. Wikipedia has numerous competent articles, math.stackexchange is overflowing with content and there's plenty additional sources available on the net. If you get stuck, do try playing around with examples of sets on paper or in a text file. That's universal advice for math.

I'll follow with some key points and some highlights of things that tripped me up while reading the book.

Axiom of extension

The axiom of extension tells us how to distinguish between sets: Sets are the same if they contain the same elements. Different if they do not.

Axiom of specification

The axiom of specification allows you to create subsets by using conditions. This is pretty much what is done every time set builder notation is employed.

Puzzled by the bit about Russell's paradox at the end of the chapter? http://math.stackexchange.com/questions/651637/russells-paradox-in-naive-set-theory-by-paul-halmos

Unordered pairs

The axiom of pairs allows one to create a new set that contains the two original sets.

Unions and intersections

The axiom of unions allows one to create a new set that contains all the members of the original sets.

Complements and powers

The axiom of powers allows one to, out of one set, create a set containing all the different possible subsets of the original set.

Getting tripped up about the "for some" and "for every" notation used by Halmos? Welcome to the club:

Using natural language rather than logical notation is commmon practice in mathematical textbooks. You'd better get used to it:

The existential quantifiers tripped me up a bit before I absorbed it. In math, you can freely express something like "Out of all possible x ever, give me the set of x that fulfill this condition". In programming languages, you tend to have to be much more... specific, in your statements.

Ordered pairs

Cartesian products are used to represent plenty of mathematical concepts, notably coordinate systems.


Equivalence relations and equivalence classes are important concepts in mathematics.


Halmos is using some dated terminology and is in my eyes a bit inconsistent here. In modern usage, we have: injective, surjective, bijective and functions that are none of these. Bijective is the combination of being both injective and surjective. Replace Halmos' "onto" with surjective, "one-to-one" with injective, and "one-to-one correspondence" with bijective.

He also confused me with his explanation of "characteristic function" - you might want to check another source there.


This chapter tripped me up heavily because Halmos mixed in three things at the same time on page 36: 1. A confusing way of talking about sets. 2. Convoluted proof. 3. n-ary cartesian product.

Families are an alternative way of talking about sets. An indexed family is a set, with an index and a function in the background. A family of sets means a collection of sets, with an index and a function in the background. For Halmos build-up to n-ary cartesian products, the deal seems to be that he teases out order without explicitly using ordered pairs. Golf clap. Try this one for the math.se treatment: http://math.stackexchange.com/questions/312098/cartesian-products-and-families

Inverses and composites

The inverses Halmos defines here are more general than the inverse functions described on wikipedia. Halmos' inverses work even when the functions are not bijective.


The axiom of infinity states that there is a set of the natural numbers.

The Peano axioms

The peano axioms can be modeled on the the set-theoretic axioms. The recursion theorem guarantees that recursive functions exist.


The principle of mathematical induction is put to heavy use in order to define arithmetic.


Partial orders, total orders, well orders -- are powerful mathematical concepts and are used extensively.

Some help on the way:

Also, keep in mind that infinite sets like subsets of w can muck up expectations about order. For example, a totally ordered set can have multiple elements without a predecessor.

Axiom of choice

The axiom of choice lets you, from any collection of non-empty sets, select an element from every set in the collection. The axiom is necessary to do these kind of "choices" with infinite sets. In finite cases, one can construct functions for the job using the other axioms. Though, the axiom of choice often makes the job easier in finite cases so it is used where it isn't necessary.

Zorn's lemma

Zorn's lemma is used in similar ways to the axiom of choice - making infinite many choices at once - which perhaps is not very strange considering ZL and AC have been proven to be equivalent.

robot-dreams offers some help in following the massive proof in the book.

Well ordering

A well-ordered set is a totally ordered set with the extra condition that every non-empty subset of it has a smallest element. This extra condition is useful when working with infinite sets.

The principle of transfinite induction means that if the presence of all strict predecessors of an element always implies the presence of the element itself, then the set must contain everything. Why does this matter? It means you can make conclusions about infinite sets beyond w, where mathematical induction isn't sufficient.

Transfinite recursion

Transfinite recursion is an analogue to the ordinary recursion theorem, in a similar way that transfinite induction is an analogue to mathematical induction - recursive functions for infinite sets beyond w.

In modern lingo, what Halmos calls a "similarity" is an "order isomorphism".

Ordinal numbers

The axiom of substitution is called the axiom (schema) of replacement in modern use. It's used for extending counting beyond w.

Sets of ordinal numbers

The counting theorem states that each well ordered set is order isomorphic to a unique ordinal number.

Ordinal arithmetic

The misbehavior of commutativity in arithmetic with ordinals tells us a natural fact about ordinals: if you tack on an element in the beginning, the result will be order isomorphic to what it is without that element. If you tack on an element at the end, the set now has a last element and is thus not order isomorphic to what you started with.

The Schröder-Bernstein theorem

The Schröder-Bernstein theorem states that if X dominates Y, and Y dominates X, then X ~ Y (X and Y are equivalent).

Countable sets

Cantor's theorem states that every set always has a smaller cardinal number than the cardinal number of its power set.

Cardinal arithmetic

Read this chapter after Cardinal numbers.

Cardinal arithmetic is an arithmetic where just about all the standard operators do nothing (beyond the finite cases).

Cardinal numbers

Read this chapter before Cardinal arithmetic.

The continuum hypothesis asserts that there is no cardinal number between that of the natural numbers and that of the reals. The generalized continuum hypothesis asserts that, for all cardinal numbers including aleph-0 and beyond aleph-0, the next cardinal number in the sequence is the power set of the previous one.

Concluding reflections

I am at the same time humbled by the subject and empowered by what I've learned in this episode. Mathematics is a truly vast and deep field. To build a solid foundation in proofs, I will now go through one or two books about mathematical proofs. I may return to Naive Set Theory after that. If anyone is interested, I could post my impressions of other mathematical books I read.

I think Naive Set Theory wasn't the optimal book for me at the stage I was. And I think Naive Set Theory probably should be replaced by another introductory book on set theory in the MIRI research guide. But that's a small complaint on an excellent document.

If you seek to get into a new field, know the prerequisites. Build your knowledge in solid steps. Which I guess, sometimes requires that you do test your limits to find out where you really are.

The next book I start on from the research guide is bound to be Computability and Logic.

Comment author: David_Kristoffersson 16 July 2015 10:14:54PM *  6 points [-]


I'm currently attempting to read through the MIRI research guide in order to contribute to one of the open problems. Starting from Basics. I'm emulating many of Nate's techniques. I'll post reviews of material in the research guide at lesswrong as I work through it.

I'm mostly posting here now just to note this. I can be terse at times.

See you there.

Comment author: David_Kristoffersson 11 July 2015 07:26:08PM *  0 points [-]

First, appreciation: I love that calculated modification of self. These, and similar techniques, can be very useful if put to use in the right way. I recognize myself here and there. You did well to abstract it all out this clearly.

Second, a note: You've described your techniques from the perspective of how they deviate from epistemic rationality - "Changing your Terminal Goals", "Intentional Compartmentalization", "Willful inconsistency". I would've been more inclined to describe them from the perspective of their central effect, e.g. something to the style of: "Subgoal ascension", "Channeling", "Embodying". Perhaps not as marketable to the lesswrong crowd. Multiple perspectives could be used as well.

Third, a question: How did you create that gut feeling of urgency?

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