Bagpipe theorem

Short description: On structure of ω-bounded connected surfaces

In mathematics, the bagpipe theorem of Peter Nyikos (1984) describes the structure of the connected (but possibly non-paracompact) ω-bounded surfaces by showing that they are "bagpipes": the connected sum of a compact "bag" with several "long pipes".

Statement

A space is called ω-bounded if the closure of every countable set is compact. For example, the long line and the closed long ray are ω-bounded but not compact. When restricted to a metric space ω-boundedness is equivalent to compactness.

The bagpipe theorem states that every ω-bounded connected surface is the connected sum of a compact connected surface and a finite number of long pipes.

A space P is called a long pipe if there exist subspaces $\{U_{\alpha }:\alpha <{\omega }_1\}$ each of which is homeomorphic to $S^1\times \mathbb{ℝ}$ such that for $n<m$ we have $\overline{U_n}\subseteq U_m$ and the boundary of $U_n$ in $U_m$ is homeomorphic to $S^1$. The simplest example of a pipe is the product $S^1\times L^{+}$ of the circle $S^1$ and the long closed ray $L^{+}$, which is an increasing union of ${\omega }_1$ copies of the half-open interval $[0,1)$, pasted together with the lexicographic ordering. Here, ${\omega }_1$ denotes the first uncountable ordinal number, which is the set of all countable ordinals. Another (non-isomorphic) example is given by removing a single point from the "long plane" $L\times L$ where $L$ is the long line, formed by gluing together two copies of $L^{+}$ at their endpoints to get a space which is "long at both ends". There are in fact $2^{{\mathrm{\aleph }}_1}$ different isomorphism classes of long pipes.

The bagpipe theorem does not describe all surfaces since there are many examples of surfaces that are not ω-bounded, such as the Prüfer manifold.

References

• Nyikos, Peter (1984), "The theory of nonmetrizable manifolds", Handbook of set-theoretic topology, Amsterdam: North-Holland, pp. 633–684