Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A paradox in Hele-Shaw displacements   We study the Hele-Shaw immiscible displacements when all surfaces tensions on
the interfaces are zero. The Saffman-Taylor instability occurs when a less
viscous fluid is displacing a more viscous one, in a rectangular Hele-Shaw
cell. We prove that an intermediate liquid with a variable viscosity can almost
suppress this instability. On the contrary, a large number of constant
viscosity liquid-layers inserted between the initial fluids gives us boundless
growth rates with respect to the wave numbers of perturbations. The same amount
of intermediate liquid is used in both cases.

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The categories of this paper are: acc-phys
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The categories of this paper are: comp-gas
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The categories of this paper are: gr-qc
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The categories of this paper are: nucl-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: math-ph
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The categories of this paper are: q-alg
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The categories of this paper are: q-bio
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The categories of this paper are: bayes-an
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The categories of this paper are: alg-geom
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The categories of this paper are: math
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The categories of this paper are: funct-an
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The categories of this paper are: atom-ph
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The categories of this paper are: supr-con
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The categories of this paper are: patt-sol
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The categories of this paper are: dg-ga