On the classification of 4-dimensional $(m,ρ)$-quasi-Einstein manifolds with harmonic Weyl curvature

In this paper we study 4-dimensional $(m,ρ)$-quasi-Einstein manifolds with harmonic Weyl curvature when $m\notin\{0,\pm1,-2,\pm\infty\}$ and $ρ\notin\{\frac{1}{4},\frac{1}{6}\}$. We prove that a non-trivial $(m,ρ)$-quasi-Einstein metric $g$ (not necessarily complete) is locally isometric to one of the followings: (i) $\mathcal{B}^2_\frac{R}{2(m+2)}\times \mathbb{N}^2_\frac{R(m+1)}{2(m+2)}$ where $\mathcal{B}^2_\frac{R}{2(m+2)}$ is a northern hemisphere in the 2-dimensional sphere $\mathbb{S}^2_\frac{R}{2(m+2)}$, $\mathbb{N}_δ$ is the 2-dimensional Riemannian manifold with constant curvature $δ$ and $R$ is the constant scalar curvature of $g$, (ii) $\mathcal{D}^2_\frac{R}{2(m+2)}\times\mathbb{N}^2_\frac{R(m+1)}{2(m+2)}$ where $\mathcal{D}^2_\frac{R}{2(m+2)}$ is one half (cut by a hyperbolic line) of the hyperbolic plane $\mathbb{H}^2_\frac{R}{2(m+2)}$, (iii) $\mathbb{H}^2_\frac{R}{2(m+2)}\times\mathbb{N}^2_\frac{R(m+1)}{2(m+2)}$, (iv) a certain singular metric with $ρ=0$, (vi) a locally conformally flat metric. By applying this local classification, we obtain a classification of complete $(m,ρ)$-quasi-Einstein manifolds under the harmonic Weyl curvature condition. Our result can be viewed as a local classification of gradient Einstein-type manifolds. One corollary of our result is the classification of $(λ,4+m)$-Einstein manifolds which can be viewed as $(m,0)$-quasi-Einstein manifolds.

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