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I need write some formula. I don't know how to choose the |space| as below:

$a_1=S_1$, $a_n=S_{n} - S_{n-1}$ |space| ($n > 1$)

(Maybe

$a_1=S_1$, $a_n=S_{n} - S_{n-1} |space| (n > 1)$

is better?)

and

\[ s_n = b_1 + b_2 + \cdots + b_n  |space|  (n\geq 1) \]

Another case:

\[ f(x)\to A, \quad g(x)\to B |space| (x\to 0) \]

I find the command \pmod in the amsmath package choosing 6mu in inline mode and 18mu in display mode. It is casual or it is traditional?

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  • You can use \quad or \qquad. Also, it is no so elegant but you can do: \begin{equation*} s_n = b_1 + b_2 + \cdots + b_n \tag{$n\geq 1$} \end{equation*}
    – Sigur
    May 21 '13 at 2:02
  • \pmod is not used to insert simple spaces. It is used to denote congruence, like 4=1 \pmod{3}.
    – Sigur
    May 21 '13 at 2:08
  • @Sigur \tag is for custom equation numbers, not for side text. May 21 '13 at 7:45
  • @AndrewSwann, yes I know. Then I wrote it is no so elegant.
    – Sigur
    May 21 '13 at 11:34
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Page 166 in the TeX book, "3. Spacing between formulas", states that the standard spacing to a side formula in a display is a \qquad:

\[ s_n = b_1 + b_2 + \cdots + b_n  \qquad  (n\geq 1) \]

It also continues

Sometimes a careless author will put two formulas next to each other in the text of the paragraph. ... Everybody who teaches proper mathematical style is agreed that formulas ought to be separated by words, not just commas;...

So in text you should be writing something like:

$a_1=S_1$ and $a_n=S_{n} - S_{n-1}$, for $n > 1$.
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  • I do the same. Use \qquad in display math (if possible), then \quad is free to use around and or similar in the formula. Kudos for including the proper way to do this in text. I generally use special macros, \qqtext, \qtextq etc. implemented in the obsious manner.
    – daleif
    May 21 '13 at 7:56
  • 1
    @daleif \quad and \qquad still work in plain text. However, I would never contemplate using them in cases like this. If rewriting with words is not possible, then the material is almost certainly meant to be in a display. Knuth's fallback suggestion is , \ May 21 '13 at 9:18
  • I totally agree, what is also why I'm glad you included that text example, especially that it is broken into smaller pieces. In displayed math I'd recommend something similar to \qquad\text{for $n>1$} not the \qquad\text{for } n>1 as one often see. Enclose the entire comment in \text (or similar), it gives a better structure to the code.
    – daleif
    May 21 '13 at 9:33
  • @Andrew Maybe I used an inappropriate example. After all, sometimes you can't add the word "for" to relieve the parentheses.
    – needhelp
    May 21 '13 at 15:20
  • "for" is not the only choice of word: "with", "while", "under", "provided",.... The point is that the mathematics should read smoothly as part of a sentence. May 21 '13 at 16:15
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For inline math I'd use the very first version you have in your question, with just a single space before the parenthesis:

$a_1=S_1$, $a_n=S_{n} - S_{n-1}$ ($n > 1$)

Sometimes the space will look a bit too small; then you can add an small space with \,. For display math I usually use \qquad (which is the same amout of space as \quad\quad):

\[ s_n = b_1 + b_2 + \cdots + b_n \qquad (n\geq 1) \]

Others prefer a \quad there – this is up to you.

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