# A symbol for the field of formal Laurent series

In my area of math one of the objects that pops up quite often is the field of all formal Laurent series. Now the trouble is that the notation for this field is $\mathbf{C} (( t ))$. Now the two brackets together is not nice. I was wondering if there is a symbol that is basically two brackets very close to each other? If not how can I make one (unfortunately I don't know metafont)?

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One cheap way is to type $\mathbf{C}(\!(t)\!)$ (and to put this into some macro):

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If \! is too much or not enough negative spacing in your font (because that's what it is: -3/18 of a quad, a quad being equal to your font-size) you could manually add spacing by adding \kern<dim>, e.g., \mathbf{C}(\kern-.2em(\ldots)\kern-.2em) –  Pieter Oct 27 '10 at 11:41
@Pieter: That is not exactly correct. \! is -3mu, and as such defined in terms of the quad width of the current math symbol font, whereas the em unit is defined by the current text font. It is better to use mu units in math. The equivalent of \! is \mkern-3mu. From that starting point, one can make adjustments until it looks right. –  Harald Hanche-Olsen Dec 16 '10 at 17:12

Some alternatives:

The stmaryrd defines \llparenthesis and \rrparenthesis, which have a line instead of a second parenthesis:

The package mathbbold defines blackboard bold parentheses, available through \Lparen and \Rparen:

As Philipp remarked, Unicode defines ⦅ U+2985 LEFT WHITE PARENTHESIS and ⦆ U+2986 RIGHT WHITE PARENTHESIS. With Xe/LuaLaTeX and unicode-math, you can use them either directly or via \lParen, \rParen. With XITS Fonts they render as

As Andrew remarks, Unicode also defines ⸨ U+2E28 LEFT DOUBLE PARENTHESIS and the corresponding closing symbol. According to the Unicode chart, they should be exactly the right symbols, but font support seems lacking (at least in the fonts I tried).

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Slightly strangely, I appear now to have U+2E28 and U+2E29 on my system in the STIX fonts, but I'm pretty sure that I didn't have them before - though that was probably a different machine. (Happened to see them whilst looking for some other character) –  Andrew Stacey Dec 16 '10 at 15:26
@Andrew: Are you sure? I can't get them to show up (in TL11 with XITS Math). –  Caramdir Aug 17 '11 at 4:06
They don't appear on the machine I'm currently on, so: No, I'm not sure. –  Andrew Stacey Aug 17 '11 at 7:58

Well, since you brought up metafont, I may as well suggest a sort of simple metafont solution.

If you knew for sure what font you were going to use, for sure, you could create the symbol by cutting and pasting the metafont def for a parenthesis and copying and shifting the 'draw' statement to your desired distance. For example, here's the metafont code for the top of the left paren for cm (from Knuth's 'bigdel.mf'):

cmchar "Extensible left parenthesis---top";
beginchar(oct"060",12u#,rule_thickness#,3dh#-rule_thickness#);
top y1=h-1; y2=-d-eps;
filldraw stroke z1e{3(x2e-x1e),y2-y1}...{down}z2e;  % upper arc
penlabels(1,2); endchar;


So you would do something like:

cmchar "Laurent double parens--top";
beginchar(oct"060",12u#,rule_thickness#,3dh#-rule_thickness#);
top y1=h-1; y2=-d-eps;
filldraw stroke z1e{3(x2e-x1e),y2-y1}...{down}z2e;  % upper arc
filldraw stroke z1e{3(x2e-x1e),y2-y1}...{down}z2e shifted(2pt);  % 2nd upper arc
penlabels(1,2); endchar;


If you're going to try this, you should check the syntax for shifting, but it's close to that. (been a while.... ;-) )

Oh, and don't mess with the original mf file. Copy it to one of your own and play with that one.

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This is not a LaTeX solution but a grammatical one. You can denote it by

\mathbb{C}\llbracket t,t^{-1}\rrbracket

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The U+FF.. block contains half- and fullwidth characters for use in Han scripts; they are not suited for mathematics. The correct characters in this case would be U+2985 and U+2986, but their glyphs are usually connected. –  Philipp Oct 27 '10 at 12:18