Riemannian geodesics of semi Riemannian warped metrics

Abstract

Let $(M_1,g_1)$ and $(M_2,g_2)$ be two $C^\infty$--differentiable connected, complete Riemannian manifolds, $k:M_1\to\mathbb R$ a $C^\infty$--differentiable function, having $0<k_0<k(x)\leq K_0$, for any $x\in M_1$ and $g:=g_1-kg_2$ the semi Riemannian metric on the product manifold $M:=M_1\times M_2$. We associate to $g$ a suitable family of Riemannian metrics $G_r+g_2$, with $r>-K_0^{-1}$, on $M$ and we call Riemannian geodesics of $g$ the geodesics of $g$ which are geodesics of a metric of the previous family, via a suitable reparametrization. Among the properties of these geodesics, we quote: For any $z_0=(x_0,y_0)\in M$ and for any $y_1\in M_2$ there exists a subset $A$ of $M_1$, such that all the geodesics of $g$ joining $z_0$ with a point $(x_1,y_1)$, with $x_1\in A$, are Riemannian. The Riemannian geodesics of $g$ determine a "partial" property of geodesic connection on $M$. Finally, we determine two new classes of semi Riemannian metrics (one of which includes some FLRM-metrics), geodesically connected by Riemannian geodesics of $g$.