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Riemannian Geometry is a subfield of Differential Geometry, which specifically studies "Riemannian Manifolds", manifolds with "Riemannian Metrics", which means that they are equipped with continuous inner products.

21 votes
Accepted

Riemannian manifold as a metric space

Isn't this the Myers-Steenrod theorem? "If $(M,g)$ and $(N,h)$ are connected Riemannian manifolds and $f:(M,d_g)\to(N,d_h)$ is an isometry, then $f:(M,g)\to(N,h)$ is a smooth isometry"
Quarto Bendir's user avatar
20 votes
0 answers
2k views

Schoen and Yau's proof of the higher dimensional positive mass theorem

In April 2017 Schoen and Yau posted on the arxiv their solution of the time-symmetric positive mass theorem in all dimensions, which has been a significant conjecture since the 70s. As of now, July 20 …
Quarto Bendir's user avatar
7 votes
Accepted

Ricci curvature : beyond heat-like flows

A small number of authors have considered hyperbolic versions of the standard flows, see e.g. "Wave character of metrics and hyperbolic geometric flow" by De-Xing Kong and Kefeng Liu and related artic …
Quarto Bendir's user avatar
5 votes
1 answer
222 views

Functions which are periodic along every geodesic

In an effort to understand some geometric rigidity theorems, I am curious about the following: let $(M,g)$ be a complete Riemannian manifold and suppose there is a nonconstant real-valued function on …
Quarto Bendir's user avatar
4 votes
0 answers
122 views

Umbilic points of minimal hypersurfaces and distributional Simons inequality

Let $\Sigma$ be a minimal hypersurface of a smooth Riemannian manifold $(M,g)$ with second fundamental form $h$. What can one say about the set $\{p\in\Sigma:h(p)=0\}$? Is each point isolated? (I feel …
Quarto Bendir's user avatar
4 votes
0 answers
812 views

History of Laplacian comparison theorem

The Laplacian comparison theorem says that if a $n$-dimensional Riemannian manifold has nonnegative Ricci curvature, then the distance function to any point satisfies $\Delta d\leq\frac{n-1}{d}$. Ther …
Quarto Bendir's user avatar
4 votes

Usage/Application of Raychaudhuri equation in Riemann geometry or pure maths

For any 1-form $\omega$ on a pseudo-Riemannian manifold, the product rule and commutation identity say that $$\omega^p\nabla_p\nabla_q\omega^q-\nabla^q(\omega^p\nabla_p\omega_q)=-\omega^p\omega^qR_{pq …
Quarto Bendir's user avatar
3 votes
0 answers
71 views

Lorentzian cobordism through the dominant energy condition

Is the answer to the following problem, or some close variant thereof, known? Briefly: Given two initial data sets $I_1=(M,g_1,k_1)$ and $I_2=(M,g_2,k_2)$, is there a time-oriented spacetime satisfyi …
Quarto Bendir's user avatar
2 votes
1 answer
180 views

Vector field along an immersion whose covariant derivative is the differential

Let $(M,g)$ be a Riemannnian manifold and let $f:\Sigma\to M$ be a smooth immersion. Then the vector bundle $f^\ast TM\to\Sigma$ has a natural bundle metric and metric-compatible connection. Can one c …
Quarto Bendir's user avatar
1 vote
1 answer
158 views

Smoothness of conformal transformations

Given a smooth pseudo-Riemannian manifold $(M,g)$ one can define the conformal group as the set of smooth diffeomorphisms $\varphi:M\to M$ such that there is a positive smooth function $u$ with $\varp …
Quarto Bendir's user avatar