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I would like to have commands for left- and right gauss brackets which have a double vertical line, but otherwise behave exactly like lfloor and rfloor. In particular, they should come in various sizes and other things should stay unchanged.

I found two candidates, which are both not optimal, unfortunately:

• llfloor and rrfloor in the package stmaryrd. Unfortunately they do not understand preceeding left and right commands. My impression is they are fixed size.




• lFloor and rFloor in the package nath. They come in different sizes, but the problem is that including the package nath introduces automatic sizing behaviour of all symbols of parantheses-type. This might be interesting, but for the moment this change is just too radical for my taste.

Thanks to Werner for providing a solution for this similar question.

A rather quick fix using scalerel would be the following.

Conveniently, scalerel provides scaleleftright[<max width>]<left obj><stuff><right obj> (and a comparable stretchleftright) for scaling/stretching both <left obj> and <right obj> to the height of <stuff> (constrained, if required and optional, to a width of <max width>). Here's a quick example:

documentclassarticle
usepackagescalerel, stmaryrd % http://ctan.org/pkg/scalerel

newcommandgaussbracket[1]stretchleftrightllfloor#1rrfloor

begindocument
[ gaussbracketdisplaystylesum_i=1^n i ]
enddocument

An option if you don't want to use any packages other than amsmath would be to place two lfloors next to each other with a little negative space between:

This has the advantage of not thickening the horizontal portion of the delimiter.

documentclassarticle
usepackageamsmath

newcommandgaussbr[1]%
 leftlfloorhspace-.33emleftlfloor #1rightrfloorhspace-.33emrightrfloor%
 

begindocument

[
gaussbrAgaussbrsum_n=0^inftya_n
]

enddocument

Of course, you can adjust the amount of negative space to your liking.