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reviewed  Approve Is g( ) rational if it looks that way on a large rational subset? 
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awarded  Enlightened 
1d

awarded  Nice Answer 
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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 nontrivial open convex subsets
(statement was trivially false for finitedimensional spaces) 
May 19 
answered  Examples of TVS with no nontrivial 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 holomorphicsymplectic 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 "HenstockKurzweil 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 
cuplength 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. 