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You want something simple like defining a set.
$$ \tag{correct} \textbf{A} = \{a, b, c, \cdots d\} $$ ...but the '{' are getting dropped...
$$ \tag{dropped} \textbf{A} = {a, b, c, \cdots d} $$
Comments
Even better would be to use
\lbraceandrbrace, since they always work properly,$$ \tag{more correct} \textbf{A} = \lbrace a, b, c, \cdots d \rbrace , $$
Even better would be to use `\lbrace` and `rbrace`, since they always work properly, $$ \tag{more correct} \textbf{A} = \lbrace a, b, c, \cdots d \rbrace , $$