Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Simple proof of a useful pointwise estimate for the wave equation   We give a simple proof of a pointwise decay estimate in 3+1 dimensions stated
in two versions, making advantage of a particular simplicity of inverting the
spherically symmetric part of the wave operator and of the comparison theorem.
We briefly explain the role of this estimate in proving decay estimates for
nonlinear wave equations or wave equations with potential terms.

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The categories of this paper are: alg-geom
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The categories of this paper are: atom-ph
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc
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The categories of this paper are: mtrl-th
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The categories of this paper are: eess
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The categories of this paper are: ao-sci
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The categories of this paper are: nucl-ex
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The categories of this paper are: nlin
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The categories of this paper are: stat
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The categories of this paper are: acc-phys
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The categories of this paper are: dg-ga
4
The categories of this paper are: cond-mat
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The categories of this paper are: q-alg
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The categories of this paper are: comp-gas