Geometric structure on a smooth manifold
In the mathematical field of differential geometry, an almost-contact structure is a certain kind of geometric structure on a smooth manifold. Such structures were introduced by Shigeo Sasaki in 1960.
Precisely, given a smooth manifold
an almost-contact structure consists of a hyperplane distribution
an almost-complex structure
on
and a vector field
which is transverse to
That is, for each point
of
one selects a codimension-one linear subspace
of the tangent space
a linear map
such that
and an element
of
which is not contained in
Given such data, one can define, for each
in
a linear map
and a linear map
by
![{\displaystyle {\begin{aligned}\eta _{p}(u)&=0{\text{ if ))u\in Q_{p}\\\eta _{p}(\xi _{p})&=1\\\varphi _{p}(u)&=J_{p}(u){\text{ if ))u\in Q_{p}\\\varphi _{p}(\xi )&=0.\end{aligned))}](https://wikimedia.org/api/rest_v1/media/math/render/svg/271b5777dd4001b4a43652ce8fb7ceec28ce81f1)
This defines a one-form
and (1,1)-tensor field
on
and one can check directly, by decomposing
relative to the direct sum decomposition
that
![{\displaystyle {\begin{aligned}\eta _{p}(v)\xi _{p}&=\varphi _{p}\circ \varphi _{p}(v)+v\end{aligned))}](https://wikimedia.org/api/rest_v1/media/math/render/svg/92af018fe9baa2a3a6d2f671d7d1fc4bfb09c225)
for any
in
Conversely, one may define an almost-contact structure as a triple
which satisfies the two conditions
for any ![{\displaystyle v\in T_{p}M}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8cde43d4dac32b2325e477be95a948eec8a77617)
![{\displaystyle \eta _{p}(\xi _{p})=1}](https://wikimedia.org/api/rest_v1/media/math/render/svg/a2c149b674763aa266b3309c0e660e0b65490cfc)
Then one can define
to be the kernel of the linear map
and one can check that the restriction of
to
is valued in
thereby defining