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May
9 |
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Question about computing group cohomology using cochains
For a group (no topology), I define the cohomology using injective resolutions in the category of all G-modules, and I prove that it can be computed using cochains (no conditions on G). For a profinite group, I define the cohomology using injective resolutions in the category of all discrete G-modules, and state that it can be computed using continuous cochains. A finite group can be regarded as a profinite group with the discrete topology, in which case the two definitions coincide (obviously). |
May
8 |
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Examples of common false beliefs in mathematics
Pete: It can screw you up very badly. For example, F-isocrystals are very different over the maximal unramified extension of Q_p and its completion. |
May
7 |
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Published results: when to take them for granted?
Actually, the signs in Deligne's Travaux de Shimura are correct. It's in his Corvallis article that he got them wrong... |
May
6 |
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Why are normal crossing divisors nice?
Hironaka proved that a smooth variety can always be realized as an open subvariety of a smooth projective variety in such a way that the boundary is a divisor with normal crossings. |
May
5 |
answered | Lifting Etale Morphisms |
Apr
18 |
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Is there a schemetical construction for modular curves over the rationals?
"I agree PVHS is the way to prove analyticity (but not evident to beginners" well, they could try reading [the next version] of my Introduction to Shimura Varieties. More seriously, this argument in the one-dimensional case is pretty trivial, and Katz and Mazur were surely aware of it, so I find the statement in your first comment a bit strong. Perhaps they considered it too obvious to write out (or just forgot). |
Apr
18 |
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Is there a schemetical construction for modular curves over the rationals?
I'm not sure I understand Brian's objections --- Shimura worked a lot with function fields and really did prove things. However, I would agree that this is completely the wrong way to do things. |
Apr
18 |
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Is there a schemetical construction for modular curves over the rationals?
From Deligne's point of view (e.g., his Corvallis article), hermitian symmetric domains are exactly the moduli spaces (in the analytic category) of certain rigidified Hodge structures, hence their quotients are also. The algebraic moduli variety carries a variation of Hodge structures of the correct type, so you get an analytic map to the quotient of the HSD (which is algebraic by Borel's theorem). Isn't that all that's going on? |
Apr
17 |
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Why are modular forms (usually) defined only for congruence subgroups?
Certainly not canonical. For example, the curve for Gamma_N classifies elliptic curves with a level N structure. Such objects have a discrete invariant, namely, an Nth root of 1 given by applying the Weil pairing to the basis. To get a connected family, you need to specify the Nth root, so this only makes sense over a field where you have one. Of course, for nonconnected curves, everything is defined over Q. |
Apr
17 |
revised |
Why are modular forms (usually) defined only for congruence subgroups?
Minor fixes. |
Apr
17 |
revised |
What algebraic group does Tannaka-Krein reconstruct when fed the category of modules of a non-algebraic Lie algebra?
Minor fixes. |
Apr
16 |
answered | Why are modular forms (usually) defined only for congruence subgroups? |
Apr
15 |
awarded | Nice Answer |
Apr
15 |
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What algebraic group does Tannaka-Krein reconstruct when fed the category of modules of a non-algebraic Lie algebra?
Thanks --- that looks like the correct way to interpret Iwahori's statements (and, yes, I should have assumed k to be algebraically closed). |
Apr
15 |
answered | What algebraic group does Tannaka-Krein reconstruct when fed the category of modules of a non-algebraic Lie algebra? |
Apr
15 |
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What algebraic group does Tannaka-Krein reconstruct when fed the category of modules of a non-algebraic Lie algebra?
There seems to be an assumption implicit in the question that when you start with an algebraic Lie algebra g, the affine group scheme G you get has Lie algebra g. As @unknown noted, this is far from true (except for semisimple Lie algebras in characteristic zero). So before trying to understand the affine group schemes you get from nonalgebraic Lie algebras, perhaps you should try to understand those you get from algebraic Lie algebras (e.g., the one-dimensional Lie algebra). |
Apr
14 |
awarded | Nice Answer |
Apr
14 |
answered | How many groups of size at most n are there? What is the asymptotic growth rate? And what of rings, fields, graphs, partial orders, etc.? |
Apr
12 |
revised |
Why does the naive definition of compactly supported étale cohomology give the wrong answer?
edited body; added 251 characters in body |
Apr
12 |
answered | Why does the naive definition of compactly supported étale cohomology give the wrong answer? |