Klein bottle
In mathematics, the Klein bottle is a certain nonorientable surface, i.e. a surface (a twodimensional topological space), for which there is no distinction between the "inside" and the "outside" of the surface. The Klein bottle was first described in 1882 by the German mathematician Felix Klein. It is closely related to the Möbius strip and embeddings of the real projective plane such as Boy's surface.
Picture a bottle with a hole in the bottom. Now extend the neck. Curve the neck back on itself, insert it through the side of the bottle (a true Klein bottle in four dimensions would not require this step, but it is necessary when representing it in threedimensional Euclidean space), and connect it to the hole in the bottom.
Unlike a drinking glass, this object has no "rim" where the surface stops abruptly. Unlike a balloon, a fly can go from the outside to the inside without passing through the surface (so there isn't really an "outside" and "inside").
Topologically, the Klein bottle can be defined as the square [0,1] × [0,1] with sides identified by the relations (0,y) ~ (1,y) for 0 ≤ y ≤ 1 and (x,0) ~ (1x,1) for 0 ≤ x ≤ 1, as in the following diagram:
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<Like the Möbius strip, the Klein bottle is a twodimensional differentiable manifold which is not orientable. Unlike the Möbius strip, the Klein bottle is a closed manifold, meaning it is a compact manifold without boundary. While the Möbius strip can be embedded in threedimensional Euclidean space R^{3}, the Klein bottle cannot. It can be embedded in R^{4}, however.
The Klein bottle can be constructed (in a mathematical sense) by joining the edges of two Möbius strips together, as described in the following anonymous limerick:
 A mathematician named Klein
 Thought the Möbius band was divine.
 Said he: "If you glue
 The edges of two,
 You'll get a weird bottle like mine."
Figure 1(a): Dissection of a Klein bottle.  Figure 1(b): ROT13 cipher inscribed along perimeter of dissection. 
In Figure 1(b), Twenty six points on the dissection's perimeter (the blue curve) have been labeled with the twenty six letters of the alphabet. But the dissection is a surface, not a curve. The red lines show how the surface is subtended by the perimeter.
Figure 2 shows a Möbius strip.
The strip is a surface: its perimeter is shown as a blue curve, and the red lines show how the surface is subtended by the perimeter.
In both the dissected Klein bottle and Möbius strip, the red lines connect letters which are related mutually in the ROT13 cipher. This helps to illustrate that half a Klein bottle is homeomorphic to a Möbius strip.
It is also possible to perceive directly that Figure 1 is a Möbius strip, by imagining that the narrower, reentrant part of the bottle no longer intersects line segment DB after the dissection is performed, but that it becomes loose from dissecting plane and Figure 1 is actually threedimensional, with line segments VW and IJ hovering above line segment DB. Then, suddenly, Figure 1 looks like a roller coaster, and by imagining the motion of a rail car along the blue rails of this roller coaster, one perceives that this roller coaster is nonorientable.
Parametrizing the Klein bottle
Since the Klein bottle is a topological surface, there can be several ways of describing it parametrically. One way is the following:

Equations (2), (4) and (6) are derivatives with respect to φ of equations (1), (3) and (5) — respectively. Functions x(φ) and y(φ) are the components of a position vector function which defines the locus of points of a plane curve which has one cusp at the origin. This curve might be called the "spine" of the Klein bottle, but this spine is not itself a subset of the Klein bottle.
Equations (3) and (5) not only describe a plane curve, they also describe the movement of a point along that plane curve. The cusp is a special point where the curve abruptly reverses direction by 180°. The movement of the point along the "spine" has been defined in such a way that the point slows down as it gets closer to the cusp, stops when it reaches the cusp, and then begins to accelerate in the opposite direction.
Functions x'(φ) and y'(φ) are the components of a vector function which is the tangent vector of spine. Functions x_{N}(φ) and y_{N}(φ) are the components of a vector function which is the unit normal vector of the spine (see FrenetSerret formulas). The binormal unit vector always points in the +z direction — since it is a curve on the xy plane — but is not necessary for parametrizing the Klein bottle. The unit normal was obtained from the tangent by rotating 90° — that is, by changing (x′, y′) to (−y′, x′) — and then normalizing. The Dirac delta terms were added to Equations (7) and (8) in order to fix discontinuities at the pinch point.
Equations (9) and (10) describe the "amplitude constant" α and the "amplitude function" w(φ). Both of these are used to modulate the "amplitude" — i.e. magnitude — of the normal vector in Equations (11), (12) and (13). The points on the Klein bottle are defined by rotating the amplified normal vector 360° around the position vector's point on the spine (the tail of the normal vector is at the head of the position vector; the point on the head of the normal vector belongs to the Klein bottle; the position vector moves along the spine). Parameter φ specifies a point on the spine, and parameter ψ specifies the degree of rotation of the normal around the spine.
See also: topology, algebraic topology
External links
 Acme Klein Bottles  Actual Klein bottles! (Or at least 3D projections of them).
 Andrew Lipson's Mathematical LEGO Sculptures  Lego constructions of Möbius strip and Klein bottle structures. This site also shows the dissection of the Klein bottle.
 Klein Bottle Images by John Sullivan