In A. L. Besse, "Einstein Manifolds", it is shown that
A 2 or 3 dimensional pseudo-Riemannian manifold is Einstein if and only if it has constant sectional curvature.
My question is if this does imply that 2, trivially, and 3 dimensional Einstein manifolds are isotropic? I think this follows by definition, but I have seen apparent contradicting assertions regarding what is an isotropic manifold and I would like to ask for clarification.
Obs: Indeed there seems to be some ambiguity with the definition of Isotropic Manifold. The one I was taught is:
A (semi) Riemannian manifold $\left(\mathcal{M},g\right)$, where $g$ the metric
tensor, is isotropic at a point $p\in\mathcal{M}$ if its sectional
curvature is a constant K for every section $\Pi\subset T_{p}\mathcal{M}$. If $\left(\mathcal{M},g\right)$ is isotropic at all
points then, it is called isotropic.
Is this definition not equivalent to the one in Wikipedia's article? Using this definition we do find that a flat torus is isotropic, as seems reasonable to expect.
Best Answer
I checked a fair number of books on Riemannian (and semi-Riemannian) geometry. Not all of these define isotropic (semi)-Riemannian manifolds, but all that do agree on the following definition (same as the Wikipedia article):
A Riemannian manifold $M$ is called isotropic if for every $x\in M$ and every pair of unit vectors $u, v\in T_xM$ there exists an isometry $f: M\to M$ fixing $x$ and sending $u$ to $v$.
A semi-Riemannian manifold $M$ is called isotropic if for every $x\in M$ and every pair of non-null vectors $u, v\in T_xM$, satisfying $<u,u>=<v,v>$, there exists an isometry $f: M\to M$ fixing $x$ and sending $u$ to $v$.
Thus, a manifold of constant curvature need not be isotropic. However, if you assume that a constant curvature Riemannian manifold is complete and simply-connected, then it is isotropic. (I am unsure about semi-Riemannian manifolds of constant curvature.)
References:
Berger, Marcel, A panoramic view of Riemannian geometry, Berlin: Springer (ISBN 3-540-65317-1/hbk). xxiii, 824 p. (2003). ZBL1038.53002.
Gallot, S.; Hulin, D.; Lafontaine, J., Riemannian geometry, Universitext. Berlin etc.: Springer-Verlag. XI, 248 p.; DM 48.00 (1987). ZBL0636.53001.
Lee, John M., Riemannian manifolds: an introduction to curvature, Graduate Texts in Mathematics. 176. New York, NY: Springer. xv, 224 p. (1997). ZBL0905.53001.
O’Neill, Barrett, Semi-Riemannian geometry. With applications to relativity, Pure and Applied Mathematics, 103. New York-London etc.: Academic Press. xiii, 468 p. {$} 45.00 (1983). ZBL0531.53051.
Wolf, J. A., Spaces of constant curvature, New York-St. Louis-San Francisco-Toronto-London-Sydney: McGraw-Hill Book Comp. XV, 408 p. (1967). ZBL0162.53304.
Edit. I can only say that the definition given to you (in lecture notes?) is nonstandard. The first part ("isotropic at a point") is consistent with the standard definitions, especially the one in Volume 3 of Spivak's book, where he uses the terminology an isotropic point rather than isotropic at a point.
However, the second part of the definition (of an isotropic manifold) given to you contradicts the standard one. What's worse, is that your definition is not equivalent to the standard one even under the assumption that the manifold is complete and simply-connected:
(a) If a connected semi-Riemannian manifold (of dimension $\ge 3$) is isotropic at each point (in the sense of your definition), then it has constant sectional curvature (Schur's theorem). But this is, of course, false in dimension 2. Applying this, together with the Poincare-Hopf theorem, one obtains that if a complete, simply-connected Riemannian manifold has dimension at least 3 and is isotropic at each point, then the manifold is isometric to one of the standard spaces of constant curvature (the Euclidean space or sphere of some radius or hyperbolic space of some constant negative curvature). From this, it follows that the manifold is isotropic in the standard sense.
(b) However, there are connected Riemannian manifolds isotropic in the standard sense which do not have constant curvature: These are various rank 1 symmetric spaces. The simplest example is the complex-projective space $CP^n$ ($n\ge 2$) with the Fubini-Study metric.
I can only say that it is regretful that your professor gave you a definition inconsistent with the standard one in the literature.
Spivak, Michael, A comprehensive introduction to differential geometry. Vol. 3, Boston, Mass.: Publish or Perish, Inc. V, 474 p. (1975). ZBL0306.53001.