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Register a new userThe Ricci tensor of almost parahermitian manifolds
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We study the pseudoriemannian geometry of almost parahermitian manifolds, obtaining a formula for the Ricci tensor of the Levi-Civita connection. The formula uses the intrinsic torsion of an underlying SL(n,R)-structure; we express it in terms of exterior derivatives of some appropriately defined differential forms. As an application, we construct Einstein and Ricci-flat examples on Lie groups. We disprove the parakahler version of the Goldberg conjecture, and obtain the first compact examples of a non-flat, Ricci-flat nearly parakahler structure. We study the paracomplex analogue of the first Chern class in complex geometry, which obstructs the existence of Ricci-flat parakahler metrics.
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We introduce a new curvature flow which matches with the Ricci flow on metrics and preserves the almost Hermitian condition. This enables us to use Ricci flow to study almost Hermitian manifolds.
We show that a basis of a semisimple Lie algebra for which any diagonal left-invariant metric has a diagonal Ricci tensor, is characterized by the Lie algebraic condition of being ``nice. Namely, the bracket of any two basis elements is a multiple of another basis element. This extends the work of Lauret and Will cite{lw13} on nilpotent Lie algebras. We also give a characterization for diagonalizing the Ricci tensor for homogeneous spaces, and study the Ricci flow behavior of diagonal metrics on cohomogeneity one manifolds.
Motivated by the recent work of Chu-Lee-Tam on the nefness of canonical line bundle for compact K{a}hler manifolds with nonpositive $k$-Ricci curvature, we consider a natural notion of {em almost nonpositive $k$-Ricci curvature}, which is weaker than the existence of a K{a}hler metric with nonpositive $k$-Ricci curvature. When $k=1$, this is just the {em almost nonpositive holomorphic sectional curvature} introduced by Zhang. We firstly give a lower bound for the existence time of the twisted K{a}hler-Ricci flow when there exists a K{a}hler metric with $k$-Ricci curvature bounded from above by a positive constant. As an application, we prove that a compact K{a}hler manifold of almost nonpositive $k$-Ricci curvature must have nef canonical line bundle.
We study an odd-dimensional analogue of the Goldberg conjecture for compact Einstein almost Kahler manifolds. We give an explicit non-compact example of an Einstein almost cokahler manifold that is not cokahler. We prove that compact Einstein almost cokahler manifolds with non-negative $*$-scalar curvature are cokahler (indeed, transversely Calabi-Yau); more generally, we give a lower and upper bound for the $*$-scalar curvature in the case that the structure is not cokahler. We prove similar bounds for almost Kahler Einstein manifolds that are not Kahler.
We describe the structure of the Ricci tensor on a locally homogeneous Lorentzian gradient Ricci soliton. In the non-steady case, we show the soliton is rigid in dimensions three and four. In the steady case, we give a complete classification in dimension three.
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