No indent in the first paragraph in a section?

I am just curious that the following format looks good to you or not? The preamble I used is just

\documentclass[12pt,a4paper]{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}

I don't feel it's so good because the first paragraph has no indent but the second does. If the first paragraph is much longer, than it's fine. But if not, it looks strange. Should I leave it, or you have suggestions for this?

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I believe paragraphs should only be indented if they come directly after another paragraph. This is just my own opinion, though, and is not always the case either. If for example, you start a paragraph without having a section heading before, the paragraph will be indented. It seems like the current indentation starts out at a non-zero value, will be set to zero when a section heading is inserted, and will be reset to the non-zero value again when a paragraph is ended. –  StrawberryFieldsForever Jan 17 at 13:01

In addition to Werner's excellent answer I'd like to make some remarks.

The indent after a section title (section is used here in a very broad sense, that is, anything with a title) is a question both of personal taste and of typographic tradition.

Tschichold, for example, states that the first indent should be suppressed only after a centered title and that all other paragraphs must be indented (see notes). The Imprimerie Nationale, which the French consider as the supreme guide in typographical matters, states that the first indent must always appear. In British (and US) typography the first indent is usually suppressed. Other national typographic styles follow one or the other trend.

Some language modules for babel change LaTeX's default of suppressing the first indent: Albanian, French, Galician, Serbian, and (Castilian) Spanish. Polyglossia extends the list: Albanian, Asturian, Basque, Breton, Catalan, Croatian, French, Galician, Greek, Interlingua, Italian, Occitan, Serbian, and (Castilian) Spanish.

The most important criterion to follow is being consistent across a document. Nobody (except perhaps in France) will hold against you a typescript where the "national tradition" isn't followed. However, breaking a well established tradition mustn't be taken lightheartedly, but also not too seriously.

The Spanish module for Babel allows for changing the style (the others don't). For Polyglossia here's what I do when I write a document in Italian:

\usepackage{polyglossia}
\setmainlanguage{italian}
\PolyglossiaSetup{italian}{indentfirst=false}

(yes, I like suppressing the first indent, also after noncentered titles).

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I like your statement However, breaking a well established tradition mustn't be taken lightheartedly, but also not too seriously. –  Sony Dec 24 '11 at 13:57
Thanks for the answer. But curious about "Nobody (except perhaps in France) .....", since I am a student in France. Why you have this opinion about France? (I am not French, so I don't know about this....) –  user565739 Dec 24 '11 at 23:08
This is only a joke: the French are known to be quite fussy about following the prescriptions by the Imprimerie Nationale (which I personally don't like at all). –  egreg Dec 25 '11 at 15:04
How can I disable babel's first paragraph indent if am using Spanish? –  Wolter Hellmund Aug 26 '13 at 0:05
@WolterHellmund In the same way: \PolyglossiaSetup{spanish}{indentfirst=false} –  egreg Aug 26 '13 at 8:29

This is perhaps very subjective.

The default for LaTeX is to have no indent after sectional headings, like \chapter and \section. The choice should be based on consistency. Do you want a paragraph indent after a sectional heading?

• Yes: Add \usepackage{indentfirst} to your document preamble. The minimalist indentfirst package sets the boolean \@afterindentfalse to (always) true.
• No: Don't change anything.

If you still don't like the consistent indent or no-indent look and you want a case-by-case control over indentation, add \indent to indent a paragraph or \noindent to remove the indent.

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When you say TeX, do you main LaTeX? or perhaps a different *TeX? Plain TeX does not define \section or \chapter so your comment doesn't seem to apply to it. –  EfForEffort Dec 6 '12 at 16:37
@D.Bueno: I was referring to the TeX engine in a generic way, so not Plain TeX. –  Werner Dec 6 '12 at 20:13
does TeX in a generic way define \chapter and \section? I just didn't understand your comment which said: "The default for TeX is to have no indent after sectional headings, like \chapter and \section" If TeX doesn't have sectional headings, it can't have a default for them, right? –  EfForEffort Dec 6 '12 at 21:20

Disclaimer: I am standing on the shoulders of giants (Werner and egreg), and probably interfering in their conversation...

Bringhurst, in The Elements of Typographic Style, v2.3, pp39-40, dedicates over a page to this, with the usual insights one has come to expect.

But he, like Tschichold and the Imprimerie Nationale, tends to be a bit prescriptive. The best explanation I've seen is given by Peter Wilson in A Few Notes on Book Design:

[...] There is also the problem when a paragraph both ends with a full line and ends a page. As the next paragraph then starts at the top of the next page, the blank line separating the two paragraphs has effectively dissappeared, thus leaving the reader in a possible state of uncertainty as to whether the paragraph continues across the page break or not. [...]

If the paragraph is the first one after a heading, then there is no need to indicate that it is a new paragraph — it is obvious from its position. So, the first paragraph after a heading need not be indented, and for some centuries now the tradition is not to indent after a heading.

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I know you're a dedicated memoir user... Anyway, the first part of the quote is why KOMA-Script's parskip=full option leaves at least one quad at the end of the line (and parskip=full+ at least the third of a full line, and ...). Not that I have ever used it as I prefer indented paragraphs as well. –  cgnieder Dec 6 '12 at 21:59
@cgnieder : I'll have to overcome my prejudice, one of these days, and try to make something with KOMA-Script. The only problem is that the day is limited to 24 hours... –  Brent.Longborough Dec 6 '12 at 23:19
Every time I look into the memoir manual I think the same the other way round :) –  cgnieder Dec 6 '12 at 23:22