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Thomas Kahle's user avatar
Thomas Kahle's user avatar
Thomas Kahle's user avatar
Thomas Kahle
  • Member for 14 years, 8 months
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Is primary decomposition still important?
A couple more remarks: (-) What A-M call modern may be old-fashioned now. Computer algebra and Groebner bases really took off after A-M's first edition. (-) Their statement is merely that primary decomposition is not 'such a central tool' anymore. One interpretation could be that it had a market share of 90% before and is now 50/50 with localization. That does not mean it's not important.
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Is primary decomposition still important?
There is probably no debate that computing minimal primes (irreducible decomposition) is useful for applications: If a biologist comes with a bunch of polynomial equations and asks you to solve them, then you compute minimal primes. The crucial words in the A-M quote are probably "in the theory". I think their statement is too strong. In the beginning of Chapter 4 of "Commutative Algebra" Eisenbud calls primary decomposition the cousin of localization.
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Why are minimal resolutions of polynomial ideals important?
In a polynomial ring over a field the resolution has length at most $n$, that is $F_{i}=0$ for $i>n$. If $I$ is homogeneous, then there exists a unique minimal free resolution that simultaneously minimizes various measures of minimality. It turns out that this minimal resolution is a direct summand of any other resolution.
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Is the sum of two prime ideals in different polynomial rings, K[X_i] and K[Y_i] a prime ideal in K[X_i Y_i]?
@Martin, that is indeed the definition of '+' for ideals (see for instance Matsumura p.1 item (4))
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Complexity of Groebner bases
Yes true, Everything I talked about is for coefficients in a field. Thanks for the reference, I'll take a look.
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Complexity of Groebner bases
clarify that coefficients should be from a field for these results
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A minimum set hitting every base of a matroid
Such a set is also called a vertex cover of the matroid/simplicial complex.
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Height of ideal in graded ring
A useful inequality for one is: In any Noetherian ring $R$ with proper ideal $I$ you have $\text{ht}(I) + \text{dim}(R/I) \leq \text{dim}(R)$.
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Height of ideal in graded ring
Hi variety, that's not a stupid question. The case only $ht(I) = ht(I')$ occurs when $I$ is already homogenous. I excluded that case in the beginning: I assumed that $p$ is non graded (and I'll also assume that $I$ is not graded, otherwise everything is trivial). We get $\text{ht}(IR_{(0)}=1$ since only the zero ideal has height zero and $IR_{(0)}$ is not the zero ideal since it contains a non-homogeneous element.
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Height of ideal in graded ring
I've updated my answer.
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Height of ideal in graded ring
More detail on the reduction to the prime case.
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What would an ideal mathematical note-taking/organizer/PIM software look like?
I think the question is too vague. Even in your new software you would need a module that does computer algebra and one that does bibliographies. How and why should they be linked? Everything except the semantic search is already there: The glue between the different "apps" is your operating system, for me Linux/text files/Emacs/shell. The semantic engine is tricky and part of ongoing research in computer science, but there are implementations like GnomeShell, MacOS Finder, and they work reasonably well if you keep all your stuff in text.
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