What is the meaning of the mathematical symbol $\{∗\}$?
The Context is the wikipedia article on the Fock Space:
Often the one particle space H is given as $L_2(X, \mu)$, the space of square-integrable functions on a space X with measure \mu
The Fock spaces then have a natural interpretation as symmetric or anti-symmetric square integrable functions as follows. Let $X^0 = \{*\}$ and $X^1 = X, X^2 = X\times X , X^3 = X \times X \times X$ etc.
Consider the space of tuples of points which is the disjoint union
$$X^* = X^0 \bigsqcup X^1 \bigsqcup X^2 \bigsqcup X^3 \bigsqcup \ldots$$
Best Answer
In this case it means "a set with a single point". Since all sets with a single point are essentially the same, we may therefore talk of "the space with a single point". Since the nature of point doesn't matter, it is denoted by $*$, and the space made up only with it is denoted by $\{ * \}$.