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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 ${\displaystyle M}$, an almost-contact structure consists of a hyperplane distribution ${\displaystyle Q}$, an almost-complex structure ${\displaystyle J}$ on ${\displaystyle Q}$, and a vector field ${\displaystyle \xi }$ which is transverse to ${\displaystyle Q}$. That is, for each point ${\displaystyle p}$ of ${\displaystyle M}$, one selects a codimension-one linear subspace ${\displaystyle Q_{p}}$ of the tangent space ${\displaystyle T_{p}M}$, a linear map ${\displaystyle J_{p}:Q_{p}\to Q_{p}}$ such that ${\displaystyle J_{p}\circ J_{p}=-\operatorname {id} _{Q_{p}}}$, and an element ${\displaystyle \xi _{p}}$ of ${\displaystyle T_{p}M}$ which is not contained in ${\displaystyle Q_{p}}$.

Given such data, one can define, for each ${\displaystyle p}$ in ${\displaystyle M}$, a linear map ${\displaystyle \eta _{p}:T_{p}M\to \mathbb {R} }$ and a linear map ${\displaystyle \varphi _{p}:T_{p}M\to T_{p}M}$ by $${\displaystyle {\begin{aligned}\eta _{p}(u)&=0{\text{ if }}u\in Q_{p}\\\eta _{p}(\xi _{p})&=1\\\varphi _{p}(u)&=J_{p}(u){\text{ if }}u\in Q_{p}\\\varphi _{p}(\xi )&=0.\end{aligned}}}$$ This defines a one-form ${\displaystyle \eta }$ and (1,1)-tensor field ${\displaystyle \varphi }$ on ${\displaystyle M}$, and one can check directly, by decomposing ${\displaystyle v}$ relative to the direct sum decomposition ${\displaystyle T_{p}M=Q_{p}\oplus \left\{k\xi _{p}:k\in \mathbb {R} \right\}}$, that $${\displaystyle {\begin{aligned}\eta _{p}(v)\xi _{p}&=\varphi _{p}\circ \varphi _{p}(v)+v\end{aligned}}}$$ for any ${\displaystyle v}$ in ${\displaystyle T_{p}M}$. Conversely, one may define an almost-contact structure as a triple ${\displaystyle (\xi ,\eta ,\varphi )}$ which satisfies the two conditions

• ${\displaystyle \eta _{p}(v)\xi _{p}=\varphi _{p}\circ \varphi _{p}(v)+v}$ for any ${\displaystyle v\in T_{p}M}$
• ${\displaystyle \eta _{p}(\xi _{p})=1}$

Then one can define ${\displaystyle Q_{p}}$ to be the kernel of the linear map ${\displaystyle \eta _{p}}$, and one can check that the restriction of ${\displaystyle \varphi _{p}}$ to ${\displaystyle Q_{p}}$ is valued in ${\displaystyle Q_{p}}$, thereby defining ${\displaystyle J_{p}}$.

• David E. Blair. Riemannian geometry of contact and symplectic manifolds. Second edition. Progress in Mathematics, 203. Birkhäuser Boston, Ltd., Boston, MA, 2010. xvi+343 pp. ISBN 978-0-8176-4958-6, doi:10.1007/978-0-8176-4959-3
• Sasaki, Shigeo (1960). "On differentiable manifolds with certain structures which are closely related to almost contact structure, I". Tohoku Mathematical Journal. 12 (3): 459–476. doi:10.2748/tmj/1178244407.