Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gorenstein Threefold Singularities with Small Resolutions via Invariant
  Theory for Weyl Groups   We classify simple flops on smooth threefolds, or equivalently, Gorenstein
threefold singularities with irreducible small resolution. There are only six
families of such singularities, distinguished by Koll{\'a}r's {\em length}
invariant. The method is to apply invariant theory to Pinkham's construction of
small resolutions. As a by-product, generators of the ring of invariants are
given for the standard action of the Weyl group of each of the irreducible root
systems.

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The categories of this paper are: stat
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The categories of this paper are: q-alg
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The categories of this paper are: math-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: chao-dyn
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The categories of this paper are: plasm-ph
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The categories of this paper are: solv-int
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The categories of this paper are: atom-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: quant-ph
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The categories of this paper are: astro-ph
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The categories of this paper are: dg-ga