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A new variational characterization of compact conformally flat 4-manifolds
Communications in Mathematics, Tome 20 (2012) no. 2, pp. 71-77 Cet article a éte moissonné depuis la source Czech Digital Mathematics Library

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In this paper, we give a new variational characterization of certain 4-manifolds. More precisely, let $R$ and $Ric$ denote the scalar curvature and Ricci curvature respectively of a Riemannian metric, we prove that if $(M^{4},g)$ is compact and locally conformally flat and $g$ is the critical point of the functional $$ F(g)=\int _{M^{4}}(aR^{2}+b|Ric|^{2})\,\mathrm {d}v_{g}\,,$$ where $$(a,b)\in \mathbb {R}^{2}\setminus L_{1}\cup L_{2}$$ $$L_{1}\colon 3a+b=0\,;\quad L_{2}\colon 6a-b+1=0\,,$$ then $(M^{4},g)$ is either scalar flat or a space form.
In this paper, we give a new variational characterization of certain 4-manifolds. More precisely, let $R$ and $Ric$ denote the scalar curvature and Ricci curvature respectively of a Riemannian metric, we prove that if $(M^{4},g)$ is compact and locally conformally flat and $g$ is the critical point of the functional $$ F(g)=\int _{M^{4}}(aR^{2}+b|Ric|^{2})\,\mathrm {d}v_{g}\,,$$ where $$(a,b)\in \mathbb {R}^{2}\setminus L_{1}\cup L_{2}$$ $$L_{1}\colon 3a+b=0\,;\quad L_{2}\colon 6a-b+1=0\,,$$ then $(M^{4},g)$ is either scalar flat or a space form.
Classification : 53C20, 53C25
Keywords: conformally flat; 4-manifold; variational characterization 
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Wu, Faen; Zhao, Xinnuan. A new variational characterization of compact conformally flat 4-manifolds. Communications in Mathematics, Tome 20 (2012) no. 2, pp. 71-77. http://geodesic.mathdoc.fr/item/COMIM_2012_20_2_a0/

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