Lie groups must be smooth manifolds, so intuition would suggest a Lie group must have an infinite number of continuously parameterized elements.
But on the other hand, I am studying Lie groups from Brian C. Hall's "Lie Groups, Lie Algebras, and Representations," in which he focuses on matrix Lie groups instead of general Lie groups. By his definition, the group $O(1)$ is a matrix Lie group. It consists of only the $1\times 1$ matrices $\{[1],[-1]\}$. Yet Hall has a theorem (Theorem 1.19) which states,
Every matrix Lie group is a smooth embedded submanifold of $M_n(\mathbb C)$ and is thus a Lie group.
So it seems either this theorem is incorrect as stated (and needs to exclude these discrete cases), or my intuition about smooth manifolds needing to be infinite is incorrect. Which of these is the case?
Best Answer
Any (countable) set $X$ admits a unique smooth $0$-manifold structure: just give $X$ the discrete topology, and take the unique map $\{x\}\to\mathbb{R}^0$ as a chart for each $x\in X$, since $\mathbb{R}^0$ is just a single point. Compatibility of the charts is trivial since no two distinct charts overlap. Moreover, any map from a $0$-dimensional smooth manifold to any smooth manifold is automatically smooth (in local charts, you just have a map $\mathbb{R}^0\to\mathbb{R}^n$ which is always smooth, since there are no partial derivatives that need to exist). In particular, any group structure on $X$ makes $X$ a Lie group.
(The countability requirement is if you require manifolds to be second-countable, which is sometimes not done but generally is in the context of Lie groups.)