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15 votes

What are examples of D-modules that I should have in mind while learning the theory?

(I'm not an expert, but you don't seem to have gotten any answers so far.) Let's start from the beginning. Let $A$ be an $n\times n$ matrix of regular functions on a Zariski open subset $X\subset \...
Donu Arapura's user avatar
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14 votes
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Do $\mathbb{A}^1-S$ and $\mathbb{A}^1-\{0,1\}$ have a finite etale cover in common?

The answer is positive if and only if $\mathbb{A}^1\setminus S$ is an arithmetic curve, i.e., $\pi_1(\mathbb{A}^1\setminus S)\subset \mathrm{Aut}(\mathbb{H}) = PSL_2(\mathbb{R})$ is an arithmetic ...
Ariyan Javanpeykar's user avatar
13 votes

How to compute the cohomology of a local system?

Suppose $X$ is a connected CW complex with fundamental group $\pi:=\pi_1(X,x)$. Then the cellular chain complex $C_*(\widetilde{X})$ of the universal cover is a chain complex of free $\mathbb{Z}\pi$-...
Mark Grant's user avatar
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9 votes
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A question regarding isomorphism in cohomology for moduli space of stable bundles over a compact Riemann surface

Things are actually simpler. View $\Gamma _n=H^1(X,\mathbb{Z}/n)$ as the group of line bundles $L\in \operatorname{Pic}^{0}(X) $ with $L^{{\tiny \otimes }n}=\mathscr{O}_X$. The map $N_0(n,k) \times \...
abx's user avatar
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8 votes

How to compute the cohomology of a local system?

In general you should have $H^1(\pi_1(X,x),V)\cong H^1(X,L)\,,$ where on the left we have the group cohomology of $\pi_1(X,x)$ acting on $V$ according to the representation. You will also have an ...
Josh Lackman's user avatar
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8 votes
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Degeneration of smooth curves and Picard-Lefschetz formula

Because the symplectic form on $H_1(C_t, \mathbb Z)$ is a perfect pairing, it suffices to check that there is a group homomorphism $H_1(C_t,\mathbb Z) \to \mathbb Z$ that sends $ \gamma$ to $1$, which ...
Will Sawin's user avatar
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6 votes
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Thurston universe gates in knots: which invariant is it?

Here is a higher-quality video of the same material. My answer is a more algebraic version of Thurston's presentation, but I will tie this back to Thurston's "intention" at the end. ...
Sam Nead's user avatar
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5 votes
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Recovering a family of rational functions from branch points

The reason for this phenomenon is that you are afflicted with a serious mathematical condition, that being: Your monodromy has monodromy. To be less cryptic, the key thing is that the fundamental ...
Will Sawin's user avatar
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5 votes

What is the mod l monodromy of a generic trigonal curve?

The paper https://arxiv.org/abs/1403.7399 shows that the monodromy of the moduli stack of trigonal curves of genus g over $\mathbb C$ is as big as possible (equal to $Sp_{2g}(\mathbb Z)$ in the ...
Aaron Landesman's user avatar
5 votes

Explicit Riemann Hilbert correspondence

It depends on what is called "explicit". If $k>2$, monodromy representation is a transcendental function of the $A_j$ and $s_j$. When $d=0$, it was expressed as an everywhere convergent power ...
Alexandre Eremenko's user avatar
4 votes
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A question about $p$-adic monodromy of abelian varieties

Let me briefly answer your question. There are two $p$-adic analogues of $R^1\pi_*\mathbb{Q}_\ell/S_0$: a (convergent) $F$-isocrystal $\mathcal E$ and an overconvergent $F$-isocrystal $\mathcal{E}^\...
Marco D'Addezio's user avatar
4 votes
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Questions about modular forms and the role of monodromy

Monodromy does not appear in such a nice way because $\omega$ is a coherent sheaf, not a locally constant sheaf. Thus there is not necessarily a way of continuing a local section along a path in $\...
Will Sawin's user avatar
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4 votes

What are examples of D-modules that I should have in mind while learning the theory?

I will edit this question as I learn more, but here's one useful example: Consider the $\mathcal{D}_{\mathbb{A}^1}$-module $$ \frac{\mathcal{D}_{\mathbb{A}^1}}{\mathcal{D}_{\mathbb{A}^1}(t\partial_t - ...
54321user's user avatar
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4 votes
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Which hyperbolic fibered knots have monodromy with a single singularity?

A fibered two-bridge knot with Seifert surface which is a plumbing of figure eight knot Seifert surfaces will have this property. One can follow the prescription of Gabai-Kazez who gave a method to ...
Ian Agol's user avatar
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4 votes
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Determine monodromy representation from local system

Here is a way to fill in the details. For simplicity, write $I = [0,1]$ for the interval, and $\exp \colon I \to S^1$ for the function $x \mapsto e^{2\pi i x}$. So let $\gamma = \exp \colon I \to S^1$ ...
R. van Dobben de Bruyn's user avatar
3 votes

Determine monodromy representation from local system

In general, I would say that there is no way around the fact that the answer uses the fact that a sheaf on $[0,1]$ is constant. Does this help? Instead of the pushforward of the constant sheaf $k$ of ...
Tom Goodwillie's user avatar
3 votes
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Reference result: proof of theorem of Kazhdan-Margulis on monodromy group of a Lefschetz pencil of odd fiber dimenion is "as big as possible"

The proof of this result is worked out in detail in page 250 (theorem 7.5) of the book Eberhard Freitag, Reinhardt Kiehl "Etale Cohomology and the Weil Conjecture" You can preview that page in ...
Myshkin's user avatar
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3 votes
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Definition of geometric monodromy

As you say, a Hurewicz fibration defines a map from $\pi_1(B)$ to the automorphisms of the fiber in the homotopy category. But a topological fiber bundle has more structure than a Hurewicz fibration. ...
Will Sawin's user avatar
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3 votes
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Čech cocycles and monodromy

First, one can pull back the Čech cocycle to S^1 and work directly with S^1 instead of X. Any two open covers have a common refinement, so it suffices to show that the monodromy map does not change ...
Dmitri Pavlov's user avatar
2 votes

How to compute the cohomology of a local system?

If you're looking for something very concrete in terms of things like explicit boundary map computations, you might be interested in looking at Section 31 of Steenrod's Topology of Fibre Bundles. As ...
Greg Friedman's user avatar
2 votes

Explicit Riemann Hilbert correspondence

You are correct that the monodromy representation is given by the $T_i$. To address your concerns about this being unique up to isomorphism, notice that a change of basis of $\mathcal{O}_X^r$ induces ...
Avi Steiner's user avatar
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2 votes
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How can I determine the monodromy of this variation of mixed hodge structures?

This monodromy is trivial because on the set $t\neq 0$ you can make a change of variables $(x,y,z)\to(x,y,z t^{-1})$, and your ideal becomes $x y (x+y+z)$, so it doesn't depend on $t$. So your family ...
Anton Mellit's user avatar
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2 votes
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Reference request: monodromy and isomorphic projections

The comments above are too long, so I am replacing them by an answer. The reference for most such results in algebraic geometry is SGA 7_2. I am sure that the following is in there somewhere. Your ...
2 votes

The monodromy in the proof of Little Picard via Klein's $J$

The usual proof of Picard's theorem along these lines used another modular function which is called $\lambda$ and which is related to $J$ by $$J=\frac{4}{27}\frac{(1-\lambda+\lambda^2)^3}{\lambda^2(1-\...
Alexandre Eremenko's user avatar
1 vote
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Tightness/Overtwistedness of genus one open book decomposition

Honda–Kazez–Matić and Baldwin proved that tight here is equivalent to right-veering, so you can sit down and work that out. The former paper shows that reducible monodromies always give you a tight ...
magicker72's user avatar
1 vote

Tightness/Overtwistedness of genus one open book decomposition

A general criterion for tightness/overtwistedness of the contact structure in terms of the page and of the monodromy of the open book was given by Andy Wand: "Tightness is preserved by Legendrian ...
Gael Meigniez's user avatar
1 vote
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Character constructed from Kummer local system lifts to representation of algebraic torus

For anyone interested: it turns out that the fundamental group in question is the étale fundamental group, which is defined as a projective limit along the automorphism groups of a pro-representing ...
Martin Skilleter's user avatar
1 vote
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The holonomy map associated to a mapping torus

If you represent the torus as $\mathbb R^2/\mathbb Z^2$, given a monodromy matrix $A \in GL_2(\mathbb Z)$, we can construct one example of the associated diffeomorphism. The multiplication-by-$A$ map $...
Will Sawin's user avatar
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