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1d
reviewed Approve Is g( ) rational if it looks that way on a large rational subset?
1d
awarded  Enlightened
1d
awarded  Nice Answer
2d
comment How bad can $\pi_1$ of a linear group orbit be?
I believe I understand how §5 and §6 show that $j^{−1}(L_vL′)$ and $j^{−1}(L_v\cap L′)$ are extensions: $$ 1→j^{−1}(L′)→j^{−1}(L_vL′)\xrightarrow{p∘j}A→1 $$ $$ 1→j^{−1}(j(M))→j^{−1}(L_v\cap L′)\xrightarrow{[\operatorname{mod}j(M)]∘j}F→1 $$ where $A=p(L_v)$ is (connected abelian Lie) × (free abelian of finite rank), and $F$ is finite. Embarrassingly I don't understand how §7 uses this to analyze $j^{−1}(L_v)$ and its $π_0$. Hopefully what I'm missing (perhaps some use of $1→L_v\cap L'→L_v→(L_vL')/L'→1?)$ was just "too obvious to bother mentioning".
May
19
revised Examples of TVS with no non-trivial open convex subsets
(statement was trivially false for finite-dimensional spaces)
May
19
answered Examples of TVS with no non-trivial open convex subsets
May
19
comment How bad can $\pi_1$ of a linear group orbit be?
Encore merci. Some (hopefully) nitpicking before I can begin to claim I understand your argument. Would you agree that: 1. In §1 of the proof, you need not really relax the hypothesis that $G$ is simply connected (for, modding out a connected subgroup does not change that)? 2. In §3, "$j(G)\subset H$" should be "$j(G)\subset L$"? 3. In §4, the very last $L$ should be $L_a$? 4. In §5, "virtually connected" should be "connected"? 5. In §6, the last sentence should be "so $\operatorname{Ker}(j)M$ has finite index in $j^{-1}(L'\cap H)$"?
May
19
reviewed Approve Bidi: A new cardinal characteristic of the continuum?
May
16
comment How bad can $\pi_1$ of a linear group orbit be?
@WillSawin: Thanks. Where you write "its $\pi_1$ vanishes" you really mean $\pi_2$, right?
May
16
comment How bad can $\pi_1$ of a linear group orbit be?
In 4th paragraph of proof: why is $\operatorname{Lie}(L)/\operatorname{Lie}(j(G))$ abelian?
May
15
reviewed Approve Expressing adj(A) as a polynomial in A?
May
15
reviewed Approve holomorphic-symplectic tag wiki excerpt
May
15
revised How bad can $\pi_1$ of a linear group orbit be?
tag added at YCor' suggestion
May
15
comment How bad can $\pi_1$ of a linear group orbit be?
Thanks!! I was secretly hoping this problem would catch your fancy :-) As a virtual beginner in algebraic groups, please give me a few hours to try and understand what you did.
May
15
awarded  Nice Question
May
14
asked How bad can $\pi_1$ of a linear group orbit be?
May
14
comment Henstock, Differentiation under the integral sign
@green113: Yes, it is. Scroll to page 1, "The purpose of this monograph is to present an exposition of a relatively new theory of the integral (variously called the "generalized Riemann integral", the "gauge integral", the "Henstock-Kurzweil integral", etc.)".
May
14
answered Henstock, Differentiation under the integral sign
May
13
reviewed Approve method of moments and Laplace transform from Shepp and Lloyd
May
11
comment cup-length of the first Chern class of complex grassmannian
@RSQ: For the (indeed, true) relation $c_1 = [\omega]$, see e.g. S. S. Chern, Complex manifolds without potential theory, p. 82.