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This tag is used if a reference is needed in a paper or textbook on a specific result.

63 votes
Accepted

"The complex version of Nash's theorem is not true"

The failure is actually more profound than you might guess at first glance: There are conformal metrics on the Poincare disk that cannot (even locally) be isometrically induced by embedding in $\ma …
Robert Bryant's user avatar
48 votes
Accepted

Hsiung on the Complex Structure of $S^6$

While it's good to have a source, such as Datta's paper that points out the error, I find that his explanation of why the key equation is wrong is not as clear as it could be. In fact, with a little …
Robert Bryant's user avatar
45 votes
Accepted

Are there some other notions of "curvature" which measure how space curves?

in addition to these excellent examples of non-local curvature quantities and their extensions to the non-smooth setting (which I am not sure the OP was anticipating), I might add the 'original' non-l …
35 votes
Accepted

A result on Lie group actions on 15-dimensional spheres?

My guess is that Weinstein was thinking of this fact, but didn't get it out correctly: For every $n\not=15$, there is a compact Lie group $H_n\subseteq\mathrm{SO}(n{+}1)$ that acts transitively on the …
Robert Bryant's user avatar
34 votes
Accepted

Why there is a relation among the second-order minors of a symmetric $4\times 4$ matrix?

This is about your specific question. For any vector space $V$ of dimension $n$, one has canonical decompositions $$ S^2(S^2V^*) \simeq S^4(V^*)\oplus K(V^*) $$ and $$ S^2(\Lambda^2V^*) \simeq \Lamb …
Robert Bryant's user avatar
30 votes
Accepted

The Lefschetz operator

There is an elementary proof in our 2003 book Exterior Differential Systems and Euler-Lagrange Partial Differential Equations (Bryant, et al, University of Chicago Press). It does not use any represen …
Robert Bryant's user avatar
24 votes
Accepted

Proofs that the conformal group in dimension $\ge 3$ is a Lie group

Answers to the OP's question depend on where the OP is willing to start. To prove that an abstractly defined group is (i.e., has the structure of) a Lie group, one will have to use something nontrivi …
Robert Bryant's user avatar
23 votes

First Explicit Irreducible Representations

If all you really want to know are the irreducible representations of ${\frak{so}}(7)$ and ${\frak{so}}(8)$ up to dimension $30$, I can save you the trouble of looking up these tables: For ${\frak{so …
Robert Bryant's user avatar
22 votes
Accepted

Is a manifold generically real analytic (with generic real analytic metric)?

Your two final questions are not the same, in spite of the "That is,..." that starts the second one, but, in the interpretations that make the most sense to me, the answer to the first is 'no' and the …
Robert Bryant's user avatar
22 votes

Probing a manifold with geodesics

There's a different kind of answer to this that you might be interested in: Suppose that, when you fire off a probe along a unit speed geodesic starting at $p\in M$, you record the direction $\theta$ …
Robert Bryant's user avatar
19 votes
Accepted

Lie algebra valued 1-forms and pointed maps to homogeneous spaces

The question you are asking is a very basic one in the theory of what Élie Cartan called "the method of the moving frame" (in the original French, "la méthode du repère mobile"), so you should be look …
Robert Bryant's user avatar
17 votes
Accepted

Where did Sophus Lie write the group commutator for two one parameter groups

Looking around on the internet, I found an English translation of Lie's 1891 paper Die Grundlagen für die Theorie der unendlichen kontinuierlichen Transformationsgruppen. I. (I.e., The foundations of …
Robert Bryant's user avatar
15 votes

Invariant differential operators on real Grassmannians

In this particular case, the invariant theory is pretty simple, so there is an explicit description. Recall that, in general, for a homogeneous space $M=G/H$, a $G$-invariant, $m$-th order linear d …
Robert Bryant's user avatar
15 votes

Reference for the proof that Möbius transformations extend to isometries of hyperbolic 3-space

Probably, you can find a discussion of this in Thurston's notes on hyperbolic 3-manifolds, or maybe some of the expositions by his students. However, what you are asking for is actually pretty simple: …
Robert Bryant's user avatar
14 votes
Accepted

Taylor expansion of the metric tensor in the normal coordinates

Using the reference https://arxiv.org/pdf/0903.2087.pdf, which agrees with https://arxiv.org/pdf/hep-th/0001078v1.pdf, which agrees with the reference U. Müller, C. Schubert and Anton M. E. van de Ven …
Robert Bryant's user avatar

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