Lecture 3 25 Figure 1.18. Life on a dodecahedron. Of course, this results in the surface intersecting itself in a circle in order to avoid this self-intersection, we could add a dimension and embed the surface in R4. Given the extra dimension to work with, we could begin with the immersion described above and perform the four-dimensional analogue of taking a string which is lying in a figure eight on a table, and lifting part of it off the surface of the table in order to avoid having it touch itself. No such manoeuvre is possible for the Klein bottle in three dimensions, but the immersion of the Klein bottle into R3 is still a popular shape, and some enterprising craftsman has been selling both ‘Klein bottles’ and beer mugs in the shape of Klein bottles at the yearly meetings of the American Math- ematical Society. We had two such glass models of Klein bottles in the class, which were bought there: one is a conventional inverted bottle very similar to the image in Figure 1.6 the other is a “Klein beer mug”, very close to a usual one in its outside shape and usable as a drinking vessel. Even when an embedding exists, it is possible for the choice of embedding to obscure certain geometric properties of an object. Con- sider the surface of a dodecahedron (or any solid, for that matter). From the point of view of the embedding in R3, there are three kinds of points on the surface a given point can lie either at a vertex, along an edge, or on a face. Being three-dimensional creatures, we see these as three distinct classes of points. Now imagine that we are two-dimensional creatures living on the surface of the dodecahedron. We can tell whether or not we are at

Purchased from American Mathematical Society for the exclusive use of nofirst nolast (email unknown) Copyright 2008 American Mathematical Society. Duplication prohibited. Please report unauthorized use to cust-serv@ams.org. Thank You! Your purchase supports the AMS' mission, programs, and services for the mathematical community.