Btukfyl

In our lecture script, we use the notion of disjoint set union. It uses a special symbol to differentiate the disjoint from the usual set union, where we add an extra dot inside of the cup symbol. Is there something like a bigudot? Or any other way to add a centered dot to any symbol?

Edit I thought amssymb provides cupdot which does what you want... but it doesn't.

usepackage{MnSymbol} provides cupdot and bigcupdot but is incompatible with amssymb which is unfortunate.

Sometimes disjoint union is depicted using sqcup which has the advantage of being in amssymb

Another possibility to go around the problem that there is no such symbol in amssymb is to use the dot-accent: dot{bigcup} or also dotbigcup. This works for all symbols, and might very well be the reason that there are no dotted symbols in amssymb.

To let TeX treat such a new construct as an operator in terms of spacing though, you need to use mathop and mathbin, that's to say mathop{dot{bigcup}} and mathbin{dot{cup}}.

Disjoint union is also sometimes written using coprod, since it is in fact the coproduct in the category of sets.

Another way is the following:

makeatletter

defmoverlay{mathpalettemov@rlay}

defmov@rlay#1#2{leavevmodevtop{%

   baselineskipz@skip lineskiplimit-maxdimen

   ialign{hfil$m@th#1##$hfilcr#2crcr}}}

newcommand{charfusion}[3][mathord]{

    #1{ifx#1mathopvphantom{#2}fi

        mathpalettemov@rlay{#2cr#3}

      }

    ifx#1mathopexpandafterdisplaylimitsfi}

makeatother



newcommand{cupdot}{charfusion[mathbin]{cup}{cdot}}

newcommand{bigcupdot}{charfusion[mathop]{bigcup}{cdot}}

The charfusion macro is built on moverlay (by D. Arsenau).

Examples:

$Acupdot B$



[ bigcupdot_{iin I} A_{i} ]

A redacted version from symbols:

newcommand{cupdot}{mathbin{mathaccentcdotcup}}

This shows @egreg's solution, "my" solution, and @Vilietha's solution side by side:

Not much difference between egreg's and mine, but maybe less to type ;-)