With a notion of tangent bundle comes the following terminology
A tangent vector on at is an element of .
The tangent space of at a point is the fiber of over ;.
A tangent vector field on is a section of .
The precise definition of tangent bundle depends on the nature of the ambient category of spaces. Below we give first the traditional definitions in ordinary differential geometry. Then we discuss the construction in more general context of smooth toposes in synthetic differential geometry and other categories of generalized smooth spaces.
There are 3 standard definitions of tangent vector known as algebraic (derivation), geometric (equivalence class of germs of curves) and physical definition (via components in local coordinate system with prescribed behaviour under change of coordinates).
Algebraically, we may define a tangent vector at on as a scalar-valued derivation on the space of germs of differentiable functions defined on near , augmented by evaluation at . That is, given partial functions and , each defined on some neighbourhood of , we have:
In light of (4), (3) is equivalent to:
Globally, we may define a tangent vector field as an ordinary (unaugmented) derivation on the space of differentiable functions defined on all of . (This works for differentiable manifolds and smooth manifolds, but not for analytic manifolds and algebraic manifolds; we need to be able to change functions locally.) That is, given functions and , we have:
In light of (4), (3) is again equivalent to:
Given a differentiable curve , the derivative of is a curve in the tangent bundle; given an argument and a function defined near , the action is given by
where indicates the usual derivative on the real line, so that is a tangent vector at . (We really only need to be defined on a neighbourhood of , of course.)
(Of course, could be replaced by any argument in this definition.)
A particularly important case is when is a level curve in some system of local coordinates at ; that is, is the point whose th coordinate is and whose other coordinates are the same as at . The local tangent vector field given by these curves may be written or (note the placement of the scripts). This is because, if a function defined on that coordinate patch is identified with a function of real variables, then becomes identified with the partial derivative . In general, a local vector field on such a coordinate patch can be written
This fact can also be turned into a definition of tanget vector.
(Yet another possible definition comes from the duality with the cotangent bundle; of course, then you have to pick a definition of that that doesn't use duality.)
Note that doesn't make sense for an arbitrary function ; it only makes sense when is given as one component of a coordinate system. That is, if and are both coordinate systems, then the two meanings of need not be the same, because constant and constant aren't the same condition. However, we can use the more complicated notation or ; this is common when is a phase space in thermodynamics. Of course, if a coordinate system is fixed by convention, then there is no ambiguity.
The above definitions in ordinary differential geometry suggest the slogan
Tangent vectors are infinitesimal curves in a space.
One of the central motivations for synthetic differential geometry is the desire to provide a context in which this slogan becomes literally formally true.
(tangent bundle in smooth toposes)
This definition captures elegantly and usefully the notion of tangent vectors as infinitesimal curves. But it is not guaranteed that the fibers of a synthetic tangent bundle are fiberwise linear, i.e. are fiberwise -modules the way one expects. Objects for which this is true are microlinear spaces in . See there for more details.
Typically, for well adapted models, under this embedding
manifolds are microlinear spaces
the synthetic definition of tangent bundle for a manifold does coincide with the ordinary notion of .
There are well-adapted smooth toposes and presented as categories of sheaves on : the first for the Grothendieck topology where covers are finite open covers, the second where covers are finite open covers and projections (MSIA, chapter VI). Both topologies are subcanonical, hence the Yoneda embedding does preserve the above property.
Hence in these models for a manifold, its ordinary tangent bundle and the full and faithful embedding, we have isomorphisms
which respect the bundle maps.
There are useful categories of generalized smooth spaces which are neither categories of manifolds nor smooth toposes modeling synthetic differential geometry. But most of them admit useful notions of tangent bundles, too, sometimes more than one.
Examples of sequences of infinitesimal and local structures
|first order infinitesimal||formal = arbitrary order infinitesimal||local = stalkwise||finite|
|derivative||Taylor series||germ||smooth function|
|tangent vector||jet||germ of curve||curve|
|square-0 ring extension||nilpotent ring extension||ring extension|
|Lie algebra||formal group||local Lie group||Lie group|
|Poisson manifold||formal deformation quantization||local strict deformation quantization||strict deformation quantization|
A textbook account of tangent bundles in the context of synthetic differential geometry is in
Further discussion of axiomatizations in this context is in
Discussion for diffeological spaces is in