This question doesn't lend itself well to plain text, so I LaTeX'd and screenshotted it. I'll do my best with the alt text.![Is there a way of writing the expression<br>\[ \sum_{n=1}^m f(n) \]<br>in a way that omits the term where $n=a$?</p><p>I know it could be written as<br>\[ \left( \sum_{n=1}^m f(n) \right) - f(a) \]<br>but this doesn't work when $f(a)$ is undefined.](https://social.jlamothe.net/photo/13528377976a61653463c9a386467268-1.png "Is there a way of writing the expression<br>\[ \sum_{n=1}^m f(n) \]<br>in a way that omits the term where $n=a$?</p><p>I know it could be written as<br>\[ \left( \sum_{n=1}^m f(n) \right) - f(a) \]<br>but this doesn't work when $f(a)$ is undefined.")
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Edit: It turns out that Friendica has a MathJax addon. Who knew?
Also, it seems like there are a number of options. The one I like best is:
$$\sum_{\begin{matrix}1 \le n \le m\\n \ne a\end{matrix}} f(n)$$
Edit 2: It looks like this doesn't render properly when federated to a server without MathJax support. I suppose I should've seen that coming.
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Darcy Casselman
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math.stackexchange.com/questio…
I'm gonna miss stack exchange.
Summation with exclusion
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imadeddinzaqout613
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Fish Face
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you will see a few variations on this if you look around. I would write it like this, but you can write basically anything unambiguous. You see notation like "a | n" or "g ∈ G" under summation symbols to represent summation over the divisors of n and elements of G, respectively, so it's pretty casual.
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Jonathan Lamothe
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