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1

If my manuscript file has

$$\biggl({\partial^2\over \partial x^2}+
   {\partial^2 \over \partial y^2}\biggr)\bigl|\varphi(x+iy)\bigr|^2 = 0$$

or

$$\bigg({\partial^2\over \partial x^2}+
  {\partial^2 \over \partial y^2}\bigg)\big|\varphi(x+iy)\big|^2 = 0$$

the result will be the same :

enter image description here

share|improve this question
to post code snippets, use 4 leading spaces, not $$. – ℝaphink May 30 '11 at 19:26
@Raphink Didn't follow you. Could you give more details ? – bellochio May 30 '11 at 19:30
1  
@Raphink $$ plain TeX for displayed math. (It's better to use \[ \] but that's a separate issue. See Why use \[...\] instead of $$ – Alan Munn May 30 '11 at 19:31
@Alan: Oh sorry, I thought that was an attempt at enforcing a code higlighting by using two $ instead of a single one. My bad ;-) – ℝaphink May 30 '11 at 19:34
See also: difference between \big[ and \bigl[ – Stefan Kottwitz♦ May 30 '11 at 20:07

2 Answers

up vote 8 down vote accepted

\big (and its friends) create an Ord(inary) atom. \bigl and \bigr (and their friends) create Open and Close atoms. Spacing between atoms can vary.

For example, TeX inserts a thin space between an Op (big operator) atom and an Ord atom, while it inserts no space between an Op atom and an Open atom.

Compare (Op and then Ord):

\[ \sum\big( \]

with (Op and then Open):

\[ \sum\bigl( \]

You can see the a list of the 13 kinds of atoms in page 158 of the TeXbook. The table in page 170 shows the spacing between pairs of adjacent atoms.

EDIT: In the example give by bellochio, there's no difference in spacing due to the kind of atoms involved in both expressions; the only part which could be considered problematic is the initial part: 

\biggl({\partial^2\over \partial x^2}

versus

\bigg({\partial^2\over \partial x^2}

However, using \showlists for the first expressions produces (only the relevant parts are shown):

### display math mode entered at line 1
\mathopen
.\hbox(14.5001+9.50012)x7.36115
..\mathon
..\hbox(14.5001+9.50012)x7.36115
...\hbox(0.39998+23.60025)x7.36115, shifted -14.10013 []
...\vbox(14.5+0.0)x0.0
...\hbox(0.0+0.0)x0.0, shifted -2.5
..\mathoff
\mathord
.\fraction, thickness = default
.\\mathord
.\.\fam1 @
.\^\fam0 2
./\mathord
./.\fam1 @
./\mathord
./.\fam1 x
./^\fam0 2

so the first atoms are of type Open, and Ord. For the second expression \showlists produces:

### display math mode entered at line 1
\mathord
.\hbox(14.5001+9.50012)x7.36115
..\mathon
..\hbox(14.5001+9.50012)x7.36115
...\hbox(0.39998+23.60025)x7.36115, shifted -14.10013 []
...\vbox(14.5+0.0)x0.0
...\hbox(0.0+0.0)x0.0, shifted -2.5
..\mathoff
\mathord
.\fraction, thickness = default
.\\mathord
.\.\fam1 @
.\^\fam0 2
./\mathord
./.\fam1 @
./\mathord
./.\fam1 x
./^\fam0 2

so the first atoms are of type Ord, and Ord. According to the table in page 170, TeX inserts no space between an Open and an Ord atom and also between two Ord atoms and that's why there's no difference in the spacing.

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From the table on page 170 we have : space between an Ord atom and an Inner atom = (1). Space between an Open atom and an Inner atom = 0. How come the spaces after the first parentheses were equal in my example, in the two cases ? – bellochio May 30 '11 at 21:30
@bellochio: I've updated my answer given an explanation. – Gonzalo Medina May 30 '11 at 23:19
I'm giving you the credit for the answer, but there is still something I didn't understand : according to your explanation, the distance between the first parenthesis an the first partial derivative should be 0. But you can see clearly that there's at least one pixel between those two atoms.How that could be explained ? – bellochio May 31 '11 at 0:30
@bellochio: I did a test boxing the symbols with a frame with \fboxsep set to 0pt and the space belong to the symbols themselves. – Gonzalo Medina May 31 '11 at 19:24
up vote 1 down vote

In that case, yes, perhaps, but try

$\log\bigl($ vs. $\log\big($

they are not the same, because \big( is AFAIR not of type math open

there are also examples regarding the type of the right fence (though I've forgotten about it at the moment)

share|improve this answer
You mean the difference is the space between the 'g' and the '(' ? Why this difference didn't show up in my example ? – bellochio May 30 '11 at 19:40

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