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Remarks on the Nijenhuis tensor and almost complex connections. (English) Zbl 0738.53014
Given a (1,1)-tensor field \(S\) the author determines all natural (1,2)- tensor fields of the same type as the Nijenhuis tensor \(N_ S\). He shows the nonexistence of affine connections polynomially naturally induced from \(S\). Also all connections \(\tilde\nabla\) naturally induced from a given symmetric affine connection and from \(S\) such that \(\hbox{Tor }\tilde\nabla=\lambda N_ S\) (\(\lambda\in R\)) are found; and conditions under which these \(\tilde\nabla\) are almost complex connections are deduced.
The paper is related to and has been motivated by a result of S. Kobayashi and K. Nomizu [Foundations of differential geometry. Vol. II. (Moskva: “Nauka” 1981; Zbl 0526.53001)] giving for every almost complex manifold with an almost complex structure \(J\) an almost complex affine connection \(\tilde\nabla\) such that \(\hbox{Tor }\tilde\nabla={1\over 8}N_ J\). The paper under review shows that besides Kobayashi and Nomizu’s example there are still many naturally induced solutions.

MSC:
53C05 Connections (general theory)
53C15 General geometric structures on manifolds (almost complex, almost product structures, etc.)
Citations:
Zbl 0526.53001
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