Flat manifold

Every one-dimensional Riemannian manifold is flat. Conversely, given that every connected one-dimensional smooth manifold is diffeomorphic to either ${\displaystyle \mathbb {R} }$ or ${\displaystyle S^{1},}$ it is straightforward to see that every connected one-dimensional Riemannian manifold is isometric to one of the following (each with their standard Riemannian structure):

• the real line
• the open interval ${\displaystyle (0,x)}$ for some number ${\displaystyle x>0}$
• the open interval ${\displaystyle (0,\infty )}$
• the circle ${\displaystyle \{(x,y)\in \mathbb {R} ^{2}:x^{2}+y^{2}=r^{2}\}}$ of radius ${\displaystyle r,}$ for some number ${\displaystyle r>0.}$

Only the first and last are complete. If one includes Riemannian manifolds-with-boundary, then the half-open and closed intervals must also be included.

The simplicity of a complete description in this case could be ascribed to the fact that every one-dimensional Riemannian manifold has a smooth unit-length vector field, and that an isometry from one of the above model examples is provided by considering an integral curve.

If ${\displaystyle (M,g)}$ is a smooth two-dimensional connected complete flat Riemannian manifold, then ${\displaystyle M}$ must be diffeomorphic to ${\displaystyle \mathbb {R} ^{2},}$ ${\displaystyle S^{1}\times \mathbb {R} ,}$ ${\displaystyle S^{1}\times S^{1},}$ the Möbius strip, or the Klein bottle. Note that the only compact possibilities are ${\displaystyle S^{1}\times S^{1}}$ and the Klein bottle, while the only orientable possibilities are ${\displaystyle \mathbb {R} ^{2},}$ ${\displaystyle S^{1}\times \mathbb {R} ,}$ and ${\displaystyle S^{1}\times S^{1}.}$

It takes more effort to describe the distinct complete flat Riemannian metrics on these spaces. For instance, ${\displaystyle S^{1}\times S^{1}}$ even has many different flat product metrics, since one could take the two factors to have different radii; hence this space even has different flat product metrics which are not isometric up to a scale factor. In order to talk uniformly about the five possibilities, and in particular to work concretely with the Möbius strip and the Klein bottle as abstract manifolds, it is useful to use the language of group actions.

Given ${\displaystyle (x_{0},y_{0})\in \mathbb {R} ^{2},}$ let ${\displaystyle T_{(x_{0},y_{0})}}$ denote the translation ${\displaystyle \mathbb {R} ^{2}\to \mathbb {R} ^{2}}$ given by ${\displaystyle (x,y)\mapsto (x+x_{0},y+y_{0}).}$ Let ${\displaystyle R}$ denote the reflection ${\displaystyle \mathbb {R} ^{2}\to \mathbb {R} ^{2}}$ given by ${\displaystyle (x,y)\mapsto (x,-y).}$ Given two positive numbers ${\displaystyle a,b,}$ consider the following subgroups of ${\displaystyle \operatorname {Isom} (\mathbb {R} ^{2}),}$ the group of isometries of ${\displaystyle \mathbb {R} ^{2}}$ with its standard metric.

• ${\displaystyle G_{e}=\{T_{(0,0)}\}}$
• ${\displaystyle G_{\text{cyl}}(a)=\{T_{(an,0)}:n\in \mathbb {Z} \}}$
• ${\displaystyle G_{\text{tor}}(a,b)=\{T_{(na,mb)}:m,n\in \mathbb {Z} \}}$ provided ${\displaystyle a<b.}$
• ${\displaystyle G_{\text{Moeb}}(a)=\{T_{(2na,0)}:n\in \mathbb {Z} \}\cup \{T_{((2n+1)a,0)}\circ R:n\in \mathbb {Z} \}}$
• ${\displaystyle G_{\text{KB}}(b)=\{T_{(2na,bm)}:n,m\in \mathbb {Z} \}\cup \{T_{((2n+1)a,bm)}\circ R:n,m\in \mathbb {Z} \}}$

These are all groups acting freely and properly discontinuously on ${\displaystyle \mathbb {R} ^{2},}$ and so the various coset spaces ${\displaystyle \mathbb {R} ^{2}/G}$ all naturally have the structure of two-dimensional complete flat Riemannian manifolds. None of them are isometric to one another, and any smooth two-dimensional complete flat connected Riemannian manifold is isometric to one of them.

There are 17 compact 2-dimensional orbifolds with flat metric (including the torus and Klein bottle), listed in the article on orbifolds, that correspond to the 17 wallpaper groups.

Note that the standard 'picture' of the torus as a doughnut does not present it with a flat metric, since the points furthest from the center have positive curvature while the points closest to the center have negative curvature. According to Kuiper's formulation of the Nash embedding theorem, there is a ${\displaystyle C^{1}}$ embedding ${\displaystyle S^{1}\times S^{1}\to \mathbb {R} ^{3}}$ which induces any of the flat product metrics which exist on ${\displaystyle S^{1}\times S^{1},}$ but these are not easily visualizable. Since ${\displaystyle S^{1}}$ is presented as an embedded submanifold of ${\displaystyle \mathbb {R} ^{2},}$ any of the (flat) product structures on ${\displaystyle S^{1}\times S^{1}}$ are naturally presented as submanifolds of ${\displaystyle \mathbb {R} ^{2}\times \mathbb {R} ^{2}=\mathbb {R} ^{4}.}$ Likewise, the standard three-dimensional visualizations of the Klein bottle do not present a flat metric. The standard construction of a Möbius strip, by gluing ends of a strip of paper together, does indeed give it a flat metric, but it is not complete.

For the complete list of the 6 orientable and 4 non-orientable compact examples see Seifert fiber space.

• Euclidean space
• Tori
• Products of flat manifolds
• Quotients of flat manifolds by groups acting freely.