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MATHEMATICS
Associated contact metric structures on the $7$-dimensional unit sphere $S^7$
Ya. V. Slavolyubova Kemerovo Institute of Plekhanov Russian University of Economics, Kemerovo, Russian Federation
Abstract:
In this paper, we construct new examples of associated contact metric structures $(\eta, \xi, \varphi, g^J)$
on the $7$-dimensional unit sphere $S^7$, other than standard.
The construction involved a Hopf bundle $\pi: S^7\to\mathbf{CP^3}$. This projection maps affinor $\varphi$ into an
almost complex structure $J$. Therefore, it became necessary to build new examples of associated
almost complex structures $J$ in the $3$-dimensional complex projective space $\mathbf{CP^3}$.
Let $\Phi$ be a nondegenerate $2$-form (a Fubini–Study form). An almost complex structure $J$ is
called positively associated with the form $\Phi$ if the following conditions are satisfied for any
vector fields $X$, $Y$:
$$
\Phi(JX, JY)=\Phi(X, Y)\text{ and }\Phi(X, JX)>0, \text{ if } X\ne0.
$$
Each positively associated almost complex structure $J$ defines a Riemannian metric $g^J$ by the
equality $g(X, Y)=\Phi(X, JY)$; the metric is also called associated. The associated metric has the
following properties:
$$
g(JX, JY)=g(X, JY),\ g(JX, Y)=\Phi(X, Y).
$$
The positively associated almost complex structure can be obtained as follows:
$$
J=J_0(1+R)(1-R)^{-1},
$$
where $R$ is a symmetric endomorphism $R: TCP^3\to TCP^3$ anticommuting with the standard
structure $J_0$,
$$
J_0=
\begin{pmatrix}
iI&0\\
0&-iI
\end{pmatrix}.
$$
In this paper, we have found a series of matrices $R$ satisfying these conditions. Each matrix of
this kind defines an associated almost complex structure in the space $\mathbf{CP^3}$. One of these matrices,
$$
R=\frac{1}{(1+|w|)^4}
\begin{pmatrix}
0& \overline{R_{\alpha}^{\overline{\beta}}}\\
R_{\alpha}^{\overline{\beta}}&0
\end{pmatrix},
$$
where the block $R_{\alpha}^{\overline{\beta}}=\begin{pmatrix}
\overline{w}^1w^2w^3 &0 &0\\
0 & w^1\overline{w}^2w^3 &0\\
0& 0& w^1w^2\overline{w}^3
\end{pmatrix}$, has been considered in more detail.
For this endomorphism, the relevant almost complex structure $J$ and a Hermite metric $g^J$ have
been found in the space $\mathbf{CP}^3$. It has been verified that the constructed structure $J$ is not integrable.
Keywords:
contact structures, associated contact metric structures, $7$-dimensional sphere.
Received: 15.12.2015
Citation:
Ya. V. Slavolyubova, “Associated contact metric structures on the $7$-dimensional unit sphere $S^7$”, Vestn. Tomsk. Gos. Univ. Mat. Mekh., 2016, no. 4(42), 44–57
Linking options:
https://www.mathnet.ru/eng/vtgu537 https://www.mathnet.ru/eng/vtgu/y2016/i4/p44
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