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There's been some recent questions on the site that are problems from math textbooks, or problems in that style.

Doubt : Pokemon Hunter and the Rogue Brook
Groups of Farmers
Cows eat too much
Who is corrupt in the Senate?
Can the car or the bike travel further?
how many times 12 men may shake hands?

Are these on topic? What do we do about them?

Related discussion: Should mathematics questions really be on-topic here?

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    $\begingroup$ Looking at how the site has changed in few years I just would like to live this small unconstructive comment here: God, people, please don't forget that puzzles are what makes your mind work, and thereby the most important part of them is LOGIC, not Story. And this is exactly what math is about. Please don't make this site to be about riddles and only riddles! $\endgroup$
    – klm123
    Commented Apr 14, 2016 at 17:22
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    $\begingroup$ I agree. Math should be a part of puzzling here. $\endgroup$
    – Rigidity
    Commented Feb 17, 2019 at 22:54

7 Answers 7

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Math puzzles are on topic, math problems are not

Let me first give some examples to illustrate the distinction I mean.

Math problems:

  • Solve for $x$: $2x+3=7$.

  • My friend gave me a riddle: She went to the store and bought some apples. Then, she went to the store and bought an equal number more apples. Then, she picked three more apples off her apples tree. Now, she has 7 apples. How many apples did she buy on her first trip?

  • At a party, every attendee has someone at the party that they know. Is it necessarily the case that there's someone at the party who knows every attendee?

  • Let $S$ be a metric space. Prove that $S$ is connected if and only if any locally-constant function from $S$ to $\mathbb{R}$ is a constant function.

I also think all the problems linked in the question are examples of math problems, though less archetypal than these examples I made up (Can the car or the bike travel further? is borderline.)

Math puzzles:

So, what makes something a math puzzle rather than math problem? I think there's a few features.

  • Clever or elegant solution, often an "aha" moment
  • Unexpected problem statement.
  • Unexpected or counterintuitive result.

For the example math puzzles (spoilers ahead):

In contrast, math problems tend to be "textbook". And by that I don't mean that they have to come from textbooks (or that textbooks can't contain math puzzles), but that they use standard, staightforward methods than anyone familiar with the subject is expected to know. They can be difficult, but their goal is to test comprehension of the material, not ingenuity. This doesn't apply to problems from math olympiads like the Putnam exam, which are designed to have clever solution.

Math problems should be closed and directed to math.SE (by the way, can we get support for migration?). I think answering these questions is well-intentioned but counterproductive, as they are liable to be homework questions. The poster doesn't learn from being given a solution, and we undermine math.SE's policies of avoiding giving full solutions and requiring the poster to show what they've tried.

Now, I intentionally chose examples that I think illustrate the two sides of the spectrum, and there's lots of grey area in between. So I'd like to see where this discussion goes.

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    $\begingroup$ I might posit that a well-known procedural solution makes something a math problem, while a little-known or self-discovered solution makes something a puzzle. (Edit: I may turn this into an answer.) $\endgroup$
    – user20
    Commented Jan 14, 2015 at 2:12
  • $\begingroup$ @Emrakul Please do write an answer. I think I have the same feeling, but don't really know how to explain what makes a solution procedural. $\endgroup$
    – xnor
    Commented Jan 14, 2015 at 2:32
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    $\begingroup$ I think you've failed to draw a distinction. At least two of the four examples you claim as puzzles are standard textbook questions to test comprehension of the material, and all of them could be used for that purpose. $\endgroup$
    – Peter Taylor
    Commented Jan 14, 2015 at 11:36
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    $\begingroup$ A major problem with math questions is that they sometimes get spectacularly bad yet high-scoring answers. If we aren't capable of making the correct answers stand out and getting the bad answers to a negative score, we have no business answering math questions. $\endgroup$
    – Gilles 'SO- stop being evil'
    Commented Jan 14, 2015 at 19:36
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    $\begingroup$ @Gilles I think that's from the dark ages of the site and hasn't been true lately. People have been aggressively downvoting bad answers. $\endgroup$
    – xnor
    Commented Jan 14, 2015 at 23:29
  • $\begingroup$ I've been thinking: if we were to add a 'math problem' close reason, would "I know it when I see it" be sufficient? $\endgroup$
    – user20
    Commented Feb 17, 2015 at 21:19
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    $\begingroup$ @Emrakul So, I think it would suffice for most cases, but would also lead to some unhappy arguments when people disagree, or when people don't want to see their question closed. $\endgroup$
    – xnor
    Commented Feb 17, 2015 at 23:56
  • $\begingroup$ @xnor That's fair, though the people who don't want their questions closed will be unhappy either way. The reason I suggest this is because it may be a place to start; the community thinks math problems shouldn't be on topic, and this might help develop better consensus. $\endgroup$
    – user20
    Commented Feb 18, 2015 at 0:06
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    $\begingroup$ @Emrakul If you think it would help make consensus, I think it would be worth trying out. An option to migrate to math.SE would also be nice. $\endgroup$
    – xnor
    Commented Feb 18, 2015 at 0:14
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    $\begingroup$ @xnor In that case, there's actually a specific reason why SE doesn't usually open migration paths. It prevents the syndrome of "well, we don't really want it, and it's sort of a math problem, so here, you guys deal with it." (This happens all too frequently with current migration paths.) See also: meta.stackexchange.com/a/203326/206222 $\endgroup$
    – user20
    Commented Feb 18, 2015 at 0:20
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    $\begingroup$ note that math textbook things and theorems are what was the most beautiful and practical puzzles for our predecessors (and children). Many math theorems still are very good puzzles (with clever and elegant solution), for those who doesn't know them. $\endgroup$
    – klm123
    Commented Apr 14, 2016 at 17:26
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    $\begingroup$ My questions (1.) and (2.) were closed for this reason. They are about using a specific form of statement to obtain arguments from it. This answer says "but that they use standard, staightforward methods that anyone familiar with the subject is expected to know".. which totally does not apply to these.. otherwise I wouldn't have asked those questions here. The proof system in question is a big mystery, and nobody knows so far, how to tackle it towards a broader range of conclusions. How to reopen? $\endgroup$
    – xamid
    Commented Mar 8 at 19:34
  • $\begingroup$ You just chose an arbitrary point on what you consider easy/expected and what you don't. I'm pretty sure the 10 numbers on a blackboard is a straightfoward problem in competition math and programming circles that pretty much everyone knows. Like on a contest it would be one of the "textbook" questions. So now it's off-topic because it's too well-known?? $\endgroup$
    – qwr
    Commented May 30 at 2:25
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I think there is quite some similarity between this question and the question posted on Magic Trick Solving. At least, my answer is very similar.

I think we will have a hard time (oh, a puzzle! Or just a challenge?...) to find hard-core rules to differentiate and will therefore often have to decided on a very subjective case-by-case basis. However, isn't it lovely that StackExchange and its voting system was build exactly to facilitated that?

I think Xnor's question gives already some very good starting points for guidelines on which everybody voting on (or posting) questions can measure his or her decision.

Guidelines for authors in self-evaluation could/should be:

What is the purpose of the question?

  • Have you found it somewhere and just want a solution? Then ask yourself:

    • is it obvious that one requires maths to solve it (and potentially non-trivial maths)? If so, if it is clearly a mathematical problem and nothing surprising, creative or fun - please post it at maths!
    • Or is it in any aspect puzzling? (i.e. would your old, dusty and joyless math's teacher just point you the door when you'd ask him the question...) Then you might be on he right site to post it! But make it clear that you really seek an answer. Otherwise it could be misinterpreted as badly written challenge...

  • Have you created it with the idea of creating a puzzle and now want to test it (and others)? If so, make sure you present it as a fun thing to solve. There are various properties which can make it qualify, just make sure at least one is fitting! (Again, fun and or the aspect of surprise or ingenuity are nice benchmarks.)

What is the presentation of the question?

  • If your (mathematical) puzzle is just maths then make sure you present it as a puzzle. There should be a reason why anybody wants to solve it. Usually fun in solving, or curiosity for a solution are the driving forces. If your posted puzzle raises neither, it is wasted effort.

  • A good puzzle can be pure maths, i.e. it is perfectly possible, that e good puzzle requires pure maths to be solved. The puzzle can be that one has to find the mathematic which is needed. Or it might be, that the maths involved is unexpected. It may also be, that the language of the puzzle simply is maths like in "combine these mathematical operators to get XY" type of puzzles.

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Having just reviewed my $?^{th}$ close vote review giving this as the reason for voting to close, I too began to wonder what the difference between a question on maths and a maths puzzle is, so that I can justify my decision at least to myself.

Then I remembered Martin Gardner and friends, and so this question has been debated before.

The difference is generally cited as mathematics versus recreational mathematics, where the difference according to Wikipedia is:

Recreational mathematics is mathematics carried out for recreation (entertainment) rather than as a strictly research and application-based professional activity.

They give a list of number theory topics that can be considered as recreational mathematics here, and a list of people considered to be recreational mathematicians here.

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I'd offer that puzzles involving any manner of mathematics should be fine here - whereas questions that are simply mathematics aren't. (This is the same conclusion as xnor, but I made it for different reasons.)

In textbook mathematics, there's a defined process for reaching a solution that's almost algorithmic for every problem. For instance: $$\int x\ln{x}\;dx$$

This isn't a puzzle. Anyone who's taken college calculus has seen the routine to solve this, and it doesn't require reasoning to determine a solution.

The criterion I propose is: a mathematics-based question is not a puzzle if there exists a common-knowledge routine for producing a solution.

In other words, a math puzzle is one which requires you to think about the process you're using. A math problem is one for which you already know the process, and simply need to figure out how to apply it.

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    $\begingroup$ I think "exists a common-knowledge routine" is the right idea, but too restrictive a criterion. A math problem can be dry and non-puzzley without belonging to a class of problems general enough for a common-knowledge routine to exist for solving it (as there is for integrals or algebraic equations). $\endgroup$
    – xnor
    Commented Jan 15, 2015 at 6:16
  • $\begingroup$ @xnor I have yet to see a strictly-math puzzle (something mathematicians might call a puzzle) on the site, so I'd be reluctant to preclude them from topicality quite yet... I do see what you mean, though. $\endgroup$
    – user20
    Commented Jan 15, 2015 at 6:17
  • $\begingroup$ I'm not sure what you mean by strictly-math puzzle then -- I would have thought my newest three questions would qualify, as would some of the examples I gave. Could you give an example? $\endgroup$
    – xnor
    Commented Jan 15, 2015 at 6:21
  • $\begingroup$ Maybe we should move to chat? (I see this becoming a more extended discussion pretty quickly.) $\endgroup$
    – user20
    Commented Jan 15, 2015 at 6:23
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    $\begingroup$ Now define "common-knowledge routine". Most people haven't taken college calculus, but a criterion which only excludes simple arithmetic isn't very useful. $\endgroup$
    – Peter Taylor
    Commented Jan 15, 2015 at 12:50
  • $\begingroup$ In retrospect, this answer is a lot less useful than I'd hoped it would be. $\endgroup$
    – user20
    Commented Jan 16, 2015 at 1:28
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    $\begingroup$ Please tell whether this one is on topic or not? The previous one also has some votes to close. $\endgroup$
    – ABcDexter
    Commented Apr 16, 2016 at 7:22
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A puzzle is the story that comes with it and the interest it creates. The idea of a puzzle is to stimulate your urge to get the result. There is a kind of eureka moment - ahhhh. It is enough to get interest of 10 people to make the puzzle a puzzle - no need of the "experts" to make judgment in such case!!!

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I believe the grey overlapping area (math puzzles math problems) is significant and, moreover, on MSE (math stack exchange) these questions are often closed as well because of different etiquette there. In fact I was once kindly asked on MSE to move such question to PSE (puzzle stack exchange).

I do not want to generalize but on PSE typically one challenges others to find answers, and one hides parts of the answer to not spoil the challenge, whereas on MSE typically one asks for help solving a problem or verifying a solution and one does not need to hide anything.

If some math questions are not welcome on ME and SE, I feel that's an unfortunate pity. In fact, then it's no longer a grey area but rather a black hole.

After all, one can use several math tags on PSE .... and one can even add puzzle tag on MSE.

Agreed, question should not be presented pure mathematical on PSE. A bit of (puzzling) story is minimum extra required ...

By the way: one could try to ask these questions on mathoverflow SE, but that's rather a research exchange and the questions we are considering here are typically not research questions but are recreational questions. And, in fact, very often on PSE the answer to the question is known by the OP.

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    $\begingroup$ There is no obligation by any Stack Exchange site, or the network as a whole, to provide space for any particular question. For example, asking for puzzle book recommendations is off-topic everywhere. Asking for how to translate something to Arabic (one of the languages which does not currently have a site), is also off-topic everywhere. Each community focuses on the questions which it chooses to deem on-topic, and no one is obligated to accept any question they do not want. It's a shame that users from other sites recommend question moves without understanding the destination site's scope. $\endgroup$
    – bobble
    Commented Sep 16 at 1:40
  • $\begingroup$ @bobbie I agree some questions don't fit well, or not al all, on any SE site. Apparently, in practice: 'math puzzles' do not seem to fit on MSE, and, 'math problems' do not seem to fit on PSE. $\endgroup$
    – FirstName LastName
    Commented Sep 16 at 12:38
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There's bound to be some level of "I know it when I see it", so I think that the best one can hope for is a set of white-listing criteria, a set of black-listing criteria, and an understanding that some questions will fall into a grey area.

Within that context, I think that one black-listing criterion should be:

If a question says words to the effect of "This is true: prove it", it belongs on maths.SE rather than here.

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    $\begingroup$ I don't think this criterion works: we accept questions of the form “I read this puzzle in a book and the solution was given as … with no explanation. Why is it that?” — but if the puzzle can be modeled mathematically then your criterion would exclude it. $\endgroup$
    – Gilles 'SO- stop being evil'
    Commented Jan 15, 2015 at 18:03
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    $\begingroup$ I think that "Prove this is true..." is just a mathy wording, and there's puzzley questions that, taken formally, are asking for a proof. This includes things like "how can first player can win the following game" or "can you connect the dots in fewer than 12 lines" or "figure out which sister is lying". $\endgroup$
    – xnor
    Commented Jan 15, 2015 at 20:12
  • $\begingroup$ @xnor, the mathy wording is more what I was getting at than the mathematical nature of the question. E.g. your planets question is phrased as a maths exercise, not as a puzzle, and as a result it feels to me to be clearly way past the blurry line. $\endgroup$
    – Peter Taylor
    Commented Jan 15, 2015 at 21:59
  • $\begingroup$ @PeterTaylor If I instead asked "Is it true that ...?", would you be OK with the question? What about "What is the maximum possible hidden area achievable?"? $\endgroup$
    – xnor
    Commented Jan 15, 2015 at 22:02
  • $\begingroup$ @xnor, I think either of those changes would move it into the grey area. $\endgroup$
    – Peter Taylor
    Commented Jan 15, 2015 at 22:04

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