Double tangent bundle

In mathematics, particularly differential topology, the double tangent bundle or the second tangent bundle refers to the tangent bundle of the total space ''TM'' of the tangent bundle of a smooth manifold ''M''
. A note on notation: in this article, we denote projection maps by their domains, e.g., ''π''_''TTM'' : ''TTM'' → ''TM''. Some authors index these maps by their ranges instead, so for them, that map would be written ''π''_''TM''.

The second tangent bundle arises in the study of connections and second order ordinary differential equations, i.e., (semi)spray structures on smooth manifolds, and it is not to be confused with the second order jet bundle.

Secondary vector bundle structure and canonical flip

Since is a vector bundle in its own right, its tangent bundle has the secondary vector bundle structure where is the push-forward of the canonical projection
In the following we denote

:$\xi = \xi^k\frac\Big, _x\in T_xM, \qquad X = X^k\frac\Big, _x\in T_xM$

and apply the associated coordinate system

:$\xi \mapsto (x^1,\ldots,x^n,\xi^1,\ldots,\xi^n)$

on ''TM''. Then the fibre of the secondary vector bundle structure at ''X''∈''T''_''x''''M'' takes the form

:$(\pi_)^_*(X) = \Big\.$

The double tangent bundle is a double vector bundle.

The canonical flip is a smooth involution ''j'':''TTM''→''TTM'' that exchanges these vector space structures
in the sense that it is a vector bundle isomorphism between and In the associated coordinates on ''TM'' it reads as

:$j\Big(X^k\frac\Big, _\xi + Y^k\frac\Big, _\xi\Big) = \xi^k\frac\Big, _X + Y^k\frac\Big, _X.$

The canonical flip has the property that for any ''f'': R² → ''M'',
:$\frac = j \circ \frac$
where ''s'' and ''t'' are coordinates of the standard basis of R ². Note that both partial derivatives are functions from R² to ''TTM''.

This property can, in fact, be used to give an intrinsic definition of the canonical flip. Indeed, there is a submersion
''p'': J²₀ (R²,M) → ''TTM'' given by
:
p( =\frac (0,0)

where ''p'' can be defined in the space of two-jets at zero because only depends on ''f'' up to order two at zero. We consider the application:
:
J: J^2_0(\mathbb^2,M) \to J^2_0(\mathbb^2,M) \quad / \quad J( = \circ \alpha
where α(''s'',''t'')= (''t'',''s''). Then ''J'' is compatible with the projection ''p'' and induces the canonical flip on the quotient ''TTM''.

Canonical tensor fields on the tangent bundle

As for any vector bundle, the tangent spaces of the fibres ''T''_''x''''M'' of the tangent bundle can be identified with the fibres ''T''_x''M'' themselves. Formally this is achieved through the vertical lift, which is a natural vector space isomorphism
defined as

:
(\operatorname_\xi X) =\frac\Big, _f(x,\xi+tX), \qquad f\in C^\infty(TM).

The vertical lift can also be seen as a natural vector bundle isomorphism

from the pullback bundle of over onto the vertical tangent bundle

:$VTM:=\operatorname(\pi_)_* \subset TTM.$

The vertical lift lets us define the canonical vector field

:$V:TM\to TTM; \qquad V_\xi := \operatorname_\xi\xi,$

which is smooth in the slit tangent bundle ''TM''\0. The canonical vector field can be also defined as the infinitesimal generator of the Lie-group action

:$\mathbb R\times (TM\setminus 0) \to TM\setminus 0; \qquad (t,\xi) \mapsto e^t\xi.$

Unlike the canonical vector field, which can be defined for any vector bundle, the canonical endomorphism

:$J:TTM\to TTM; \qquad J_\xi X := \operatorname_\xi(\pi_)_*X, \qquad X\in T_\xi TM$

is special to the tangent bundle. The canonical endomorphism ''J'' satisfies

:
\operatorname(J)=\operatorname(J)=VTM, \qquad \mathcal L_VJ= -J, \qquad J ,YJ X,YJ ,JY

and it is also known as the tangent structure for the following reason. If (''E'',''p'',''M'') is any vector bundle
with the canonical vector field ''V'' and a (1,1)-tensor field ''J'' that satisfies the properties listed above, with ''VE'' in place of ''VTM'', then the vector bundle (''E'',''p'',''M'') is isomorphic to the tangent bundle of the base manifold, and ''J'' corresponds to the tangent structure of ''TM'' in this isomorphism.

There is also a stronger result of this kind which states that if ''N'' is a 2''n''-dimensional manifold and if there exists a (1,1)-tensor field ''J'' on ''N'' that satisfies

:
\operatorname(J)=\operatorname(J), \qquad J ,YJ X,YJ ,JY

then ''N'' is diffeomorphic to an open set of the total space of a tangent bundle of some ''n''-dimensional manifold ''M'', and ''J'' corresponds to the tangent structure of ''TM'' in this diffeomorphism.

In any associated coordinate system on ''TM'' the canonical vector field and the canonical endomorphism have the coordinate representations

:$V = \xi^k\frac, \qquad J = dx^k\otimes\frac.$

(Semi)spray structures

A Semispray structure on a smooth manifold ''M'' is by definition a smooth vector field ''H'' on ''TM'' \0 such that ''JH''=''V''. An equivalent definition is that ''j''(''H'')=''H'', where ''j'':''TTM''→''TTM'' is the canonical flip. A semispray ''H'' is a spray, if in addition, 'V'',''H''''H''.

Spray and semispray structures are invariant versions of second order ordinary differential equations on ''M''. The difference between spray and semispray structures is that the solution curves of sprays are invariant in positive reparametrizations{{jargon-inline, reason=What makes a reparametrization positive?, date=September 2015 as point sets on ''M'', whereas solution curves of semisprays typically are not.

Nonlinear covariant derivatives on smooth manifolds

The canonical flip makes it possible to define nonlinear covariant derivatives on smooth manifolds as follows. Let
:$T(TM\setminus 0) = H(TM\setminus 0) \oplus V(TM\setminus 0)$
be an Ehresmann connection on the slit tangent bundle ''TM''\0 and consider the mapping
:$D:(TM\setminus 0)\times \Gamma(TM) \to TM; \quad D_XY := (\kappa\circ j)(Y_*X),$
where ''Y''_*:''TM''→''TTM'' is the push-forward, ''j'':''TTM''→''TTM'' is the canonical flip and κ:''T''(''TM''/0)→''TM''/0 is the connector map. The mapping ''D''_''X'' is a derivation in the module Γ (''TM'') of smooth vector fields on ''M'' in the sense that

* $D_X(\alpha Y + \beta Z) = \alpha D_XY + \beta D_XZ, \qquad \alpha,\beta\in\mathbb R$.
* D_X(fY) = X + f D_XY, \qquad \qquad \qquad f\in C^\infty(M).

Any mapping ''D''_''X'' with these properties is called a (nonlinear) covariant derivative
I.Bucataru, R.Miron, ''Finsler-Lagrange Geometry'', Editura Academiei Române, 2007. on ''M''.
The term ''nonlinear'' refers to the fact that this kind of covariant derivative ''D''_''X'' on is not necessarily linear with respect to the direction ''X''∈''TM''/0 of the differentiation.

Looking at the local representations one can confirm that the Ehresmann connections on (''TM''/0,π_''TM''/0,''M'') and nonlinear covariant derivatives on ''M'' are in one-to-one correspondence. Furthermore, if ''D''_''X'' is linear in ''X'', then the Ehresmann connection is linear in the secondary vector bundle structure, and ''D''_''X'' coincides with its linear covariant derivative.

See also

* Spray (mathematics)
* Secondary vector bundle structure
* Finsler manifold

References

Differential geometry
Topology