[OS X TeX] OT: spacing after operator in math

[Bruno Voisin]

I have been trying to determine how is specified the spacing between  
a log-like operator (\log, \sin, etc.) and its argument, with no  
luck. I imagine it must be some value in math units (mu), but even  
after browsing through chs 17 ff. of the TeXbook I feel none the wiser.

For defining a log-like operator you can either use the amsmath  
package and the \DeclareMathOperator command as in:

\DeclareMathOperator{\sign}{sign}

or LaTeX "primitives" as in:

\newcommand{\sign}{\mathop{\mathgroup\symoperators sign}\nolimits}

As far as I can tell, the fact that \sign is defined as an operator  
(\mathop) ensures that some space will be added before its argument.  
I'm seeing journal-specific macros which use instead:

\newcommand{\sign}{\mbox{sign}}

which I think is wrong. In particular, that would prevent the above  
standard space to be added before the argument. I have seen also  
stuff like \newcommand{\sign}{\mbox{sign}\;}, which doesn't feel  
right either.

But what the heck is this space? Is this some hard-coded value inside  
the source code of TeX itself, or some accessible and clearly defined  
parameter within plain TeX or LaTeX?

Bruno Voisin

(I know this is OT, not Mac-specific, but I'm trying here first  
before subscribing to a general-purpose TeX list just for this, in  
case some knowledgeable person here would know the answer already.)

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