A symbol for the field of formal Laurent series

Question

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)?

Answer 1

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

Answer 2

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).

Answer 3

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#);
adjust_fit(4u#,-.25u#); pickup fine.nib;
numeric min_breadth,max_breadth;
min_breadth=rule_thickness+.6dw; max_breadth=bold+2dw;
pos1(hround min_breadth,0); pos2(hround max_breadth,0);
rt x1r=hround(w-1.25u+.5min_breadth); lft x2l=hround 1.25u;
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#);
adjust_fit(4u#,-.25u#); pickup fine.nib;
numeric min_breadth,max_breadth;
min_breadth=rule_thickness+.6dw; max_breadth=bold+2dw;
pos1(hround min_breadth,0); pos2(hround max_breadth,0);
rt x1r=hround(w-1.25u+.5min_breadth); lft x2l=hround 1.25u;
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.

Answer 4

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

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

Answer 5

Looking through the unicode characters, I found U+FF5F and U+FF60 which are ｟ and ｠ respectively. Unfortunately, my attempts to put these into a LaTeX file didn't work, probably because I'm not sufficiently unicode-aware yet. But if some kind soul comes along and explains how to do it, you would be able to produce C｟x｠ to your heart's content.

Hmm, according to the description this is a "fullwidth left white parenthesis". There's also a "left double parenthesis" which is U+2E28, but that doesn't show up with my fonts: ⸨.

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(Update: bizarrely, the ⸨ and ⸩ have just appeared on my system in the STIX fonts. Unfortunately, I'm not sure which version of the fonts I'm using.)