There was recently a question about calculation short-cuts to save time. I'd like to know what the community consensus is about the acceptability of such questions here.

I myself voted to close -- I'd say this has more to do with mental tricks than mathematics. I don't expect questions here to be MO-level, of course (in fact, I expect them not to be), but I think we should restrict ourselves to questions that are about doing and using mathematics.

Incidentally, I don't mind questions such as this one in moderation. This isn't a site about math education, but I don't see a problem with a few such questions, especially since it seems more serious (with a definite and productive purpose) than the previous one.


What I'm currently doing on the main site is abstaining from closing anything that is not obviously off-topic. As an MO user, I am probably biased against the kind of questions the typical MU user would find interesting, so I don't want to decide closing policy until I have a much better sense of what the community here wants.

Why do you not consider yourself part of the community here too? – Larry Wang Jul 29 '10 at 23:32
I just don't want to override the majority. There have been discussions about this kind of stuff at meta.MO. – Qiaochu Yuan Jul 29 '10 at 23:36
Screw the majority, Qiaochu. This is the time set precedents. Worry about politics later. – 97832123 Jul 29 '10 at 23:39
On MO some people feel that a small group of high-rep users are voting to close many questions that others would like to see open. (It only takes an oligarchy of 5 to do this.) I would prefer to err on the opposite side here. – Qiaochu Yuan Jul 30 '10 at 1:52

I would ask to make this community wiki. Some of the tricks could be interesting

Especially since ideally, people will be explaining why these tricks work. – Larry Wang Jul 29 '10 at 23:25
The "why these tricks work" part is something I'd consider to be mathematics. – Isaac Jul 29 '10 at 23:55
And, of course, "how can this be generalized". – Charles Oct 5 '10 at 17:55

I can't understand why you would say calculation short-cuts are not mathematics.

Here is an example: Prove there is no power of two which has all the digits from 1 to 9 in it the same number of times. To prove this we use a calculation short-cut called casting out nines.

Maybe I have misunderstood the question.

So, your proof of this property of powers of two will rest on the "calculation short-cut called casting out nines" ? That proof would not belong on this site without a reason to believe the "calculation shortcut". – Tom Stephens Jul 30 '10 at 19:27
  1. Calculation, including simple arithmetic, is part of mathematics, and/or a use of mathematics. There are books written about it, and published and sold even in this age of computers. (In bookstores and libraries these books are in the math section.)

  2. "Are calculation tricks a part of mathematics?" is a valid math.SE question to establish correctness of #1.

  3. "Is 'is a part of mathematics?' a valid math.SE question?" is a valid meta question to establish correctness of #2.

  4. There is no meta.meta.math.SE for #3.


I would have voted to close, too.


From the FAQ:

Mathematics - Stack Exchange is for people studying math at any level & professionals in related fields.

If we really mean the above then mental tricks (and understanding why they work) are an essential part of any mathematicians development and questions on them should be acceptable, but I'd prefer to see more focussed questions of the form "Why does [trick] work?" or "Are there shortcuts to calculating [specific thing]".

Yeah, the real problem here is that it's not a specific question, not that it's too easy or "soft". – ShreevatsaR Jul 30 '10 at 17:10

I voted to close too, but some trick do have a mathematical content.

For example, 32*28 may be seen as 302-4, using the fact that (a+b)(a-b)=a2-b2.


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