I don't think that's true. If the dimension of M is bigger than that of P, then a necessary condition is that every tangent hyperplane PsubsetTpM develops locally into a totally geodesic submanifold of M. This is not true for arbitrary manifolds. Some examples of manifolds in which this is true include those of constant sectional curvature. Symmetric spaces being a special subclass which verifies this conditions. Someone more awake can probably come up with a sufficient criterion.
I can't think of a very good example right now, but I am pretty sure that if take the 3-dimensional Riemannian Schwarzschild solution, start from a r-orthogonal plane outside of the apparent horizon, you'd get a counterexample.
If the dimension of P is the same as dimension of M, however, then you should be okay as long as you make U small enough.
Edit: Ah, for P 2 dimensional and M 3, by applying the Codazzi equations one sees that a necessary condition for locally developing the hyperplanes to totally geodesic submanifolds is that Ric(X,Y)=0 whenever g(X,Y)=0. This is obviously a very strong condition that is not satisfied by most manifolds.
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