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In this question here the OP asks for hints for a problem rather than a full proof.

Proof of subfactorial formula $!n = n!- \sum_{i=1}^{n} {{n} \choose {i}} \quad!(n-i)$

Now, while I would like to respect that request, I also feel that questions on this site are not intended just for the OP's benefit. This leads me to the question...

Question: Is there any way to use some form of spoiler space, so that it's possible to post the answer for the other readers' benefit, but at the same time hiding it from those who do not want it?

My attempted "look at the previous version of this post" turned out a disaster. I've seen people use rot13, but that seems like a lot of fuss (and clashes with the mathematics).

On some sites they use white text on white background for spoily material, which, when you select with the mouse, reveals the text. Is that possible?


Testing:

! Spoiler Space

! More spoiler space


Spoiler Space More spoiler space


What happens if I write a really long sentence. Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test Test


Maybe it'll involve some maths like $E=mc^2$ or exclamation marks $n!=n \times (n-1)!$.

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    $\begingroup$ Excellent question, Douglas! $\endgroup$ – Mike Spivey Dec 16 '10 at 5:42
  • $\begingroup$ This is interesting, it might make a good compromise for answers to homework questions that are a bit spoily. $\endgroup$ – Douglas S. Stones Dec 16 '10 at 5:42
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    $\begingroup$ Man, now if only we could preview spoiler blocks, we'd be all set... $\endgroup$ – J. M. is a poor mathematician Dec 16 '10 at 6:36
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    $\begingroup$ Hmm... since I don't have enough reputation to edit I don't know what are you guys doing for this? Any help? $\endgroup$ – Quixotic Dec 16 '10 at 13:42
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    $\begingroup$ @Deb: See this... $\endgroup$ – J. M. is a poor mathematician Dec 16 '10 at 13:48
  • $\begingroup$ @J.M:That helped thanks :-) $\endgroup$ – Quixotic Dec 16 '10 at 13:51
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Good idea. I searched meta.stackoverflow.com to see if this had already come up, and I found this, showing that it was recently implemented. I'm going to try it:

$$!n = n!- \sum_{i=1}^{n} {{n} \choose {i}} \quad!(n-i)$$

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  • $\begingroup$ Hmmm... it works here, but I didn't manage to get it working over at the parent site (it just highlights the text and puts an ! at the start of the sentence). $\endgroup$ – Douglas S. Stones Dec 16 '10 at 5:31
  • $\begingroup$ Okay, worked it out... it doesn't like two >!'s separated by an empty line. $\endgroup$ – Douglas S. Stones Dec 16 '10 at 5:36
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    $\begingroup$ And thank you for finding this out, Jonas. I will probably be trying it out myself. $\endgroup$ – Mike Spivey Dec 16 '10 at 5:44
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    $\begingroup$ This is awesome. Maybe we should be putting something about this in the FAQ. This certainly should help towards the problem of how to handle homework! $\endgroup$ – J. M. is a poor mathematician Dec 16 '10 at 6:03
  • $\begingroup$ I hope we don't bump up too many old questions from our editing previous answers to have spoilers... ;P $\endgroup$ – J. M. is a poor mathematician Dec 16 '10 at 6:15
  • $\begingroup$ What an odd implementation of the idea... With so much 2.0ishness couldn't they just put a button or something? Text appearing and disappearing because I'm moving the pointer from one side of my screen to another and the thing happens to be in the middle... Great! $\endgroup$ – Mariano Suárez-Álvarez Dec 16 '10 at 6:18
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    $\begingroup$ @J.M.: I bumped one, mainly because I had already been tempted to post a spoiler after waiting a while to let the OP figure out the details. This just gave me an excuse :) $\endgroup$ – Jonas Meyer Dec 16 '10 at 6:18
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    $\begingroup$ I was itching to edit in a complete solution to one of my previous answers, but I didn't want to easily give the whole thing away to the OP. Now I'd better have a look at that answer again... XD $\endgroup$ – J. M. is a poor mathematician Dec 16 '10 at 6:23
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Here is a sample bit of LaTeX contained in a spoiler block:

$\sum_{n=1}^\infty\frac{1}{n^2}=\frac{\pi^2}{6}$
$$\sum_{n=1}^\infty\frac{1}{n^2}=\frac{\pi^2}{6}$$

This is the code I used to produce it:

  >! $\sum_{n=1}^\infty\frac{1}{n^2}=\frac{\pi^2}{6}$  
  >! $$\sum_{n=1}^\infty\frac{1}{n^2}=\frac{\pi^2}{6}$$ 

However, when I moused over the spoiler block in the preview area while composing this post:

enter image description here

So, displayed equations don't appear displayed in the preview area, although they come out just fine in the final product.

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Some more testing:

1.

italicized text goes here

2.

bold text goes here

3.

Let's try $\LaTeX$ italicized text bold text and hyperlinks.

4.

$$\color{red}{\text{Does}}\;\color{green}{\text{colored}}\;\color{blue}\LaTeX\;\color{yellow}{\text{work?}}$$ But black text and $\LaTeX$ should still be obscured.

I'll edit this when I think of more stress tests.

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    $\begingroup$ >! But it doesn't work in comments. $\endgroup$ – J. M. is a poor mathematician Dec 16 '10 at 6:20
  • $\begingroup$ @JM: $\color{white}{\text{spoiler}}$ $\endgroup$ – kennytm Dec 16 '10 at 18:30
  • $\begingroup$ @Kenny: Yeah, I know about that; I'm just demonstrating that the functionality doesn't work in comments. ;) $\endgroup$ – J. M. is a poor mathematician Dec 17 '10 at 0:10
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What may be needed is a means by which an answer is visible to everyone except the OP.

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    $\begingroup$ Since OPs can trivially log out and view the question and its answers as a visitor, and since they can trivially have two accounts, going to such extremes is pretty pointless. We should never underestimate the amount of work people will do to avoid work... $\endgroup$ – Mariano Suárez-Álvarez Dec 16 '10 at 22:30
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    $\begingroup$ Perhaps the answer would only be visible to users (other than the OP) who also have a certain minimal reputation level? $\endgroup$ – JDH Dec 17 '10 at 1:07
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    $\begingroup$ Meanwhile, my actual view is that full solutions to homework questions are no problem and actually good for the site, as long as the quesions and answers are interesting. The only problematic issue I see with homework is that the problems might be boring for most users. I would enjoy seeing some very good homework problems! $\endgroup$ – JDH Dec 17 '10 at 1:09
  • $\begingroup$ The OP can always view the source (e.g. meta.math.stackexchange.com/revisions/…) $\endgroup$ – kennytm Dec 17 '10 at 10:16
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I would very much prefer that we did not use this feature. It is simply too distracting.

I am really surprised that this has been deemed accetable UI!

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  • $\begingroup$ I wasn't a fan of it either, but people love their spoilers... it came up on multiple sites with wildly different backgrounds, and got upvoted a lot. $\endgroup$ – Jeff Atwood Dec 17 '10 at 5:32
  • $\begingroup$ @Jeff: while I can understand why some people would like having some kind of UI for this, did the actual implemenation get many votes? If so, well, this is just another example of the universal truth that design is best not be done by polling... $\endgroup$ – Mariano Suárez-Álvarez Dec 20 '10 at 0:04

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