diff --git a/theorems/T000913.md b/theorems/T000913.md new file mode 100644 index 000000000..dc8c39857 --- /dev/null +++ b/theorems/T000913.md @@ -0,0 +1,17 @@ +--- +uid: T000913 +if: + and: + - P000168: true + - P000052: false +then: + P000244: false +--- + +Let $x$ be a non-isolated point in $X$ +and let $\mathscr V$ be a countable collection of nonempty open sets. +To show $X$ has uncountable $\pi$-character, we show that $\mathscr V$ is not a local $\pi$-base for $x$. +Each $V\in\mathscr V$ contains a point $x_V\ne x$. +One characterization of {P168} is that every countable set is closed. +So the countable set $F:=\{x_V:V\in\mathscr V\}$ is closed in $X$. +Its complement $X\setminus F$ is an open neighborhood of $x$ not containing any $V\in\mathscr V$.