Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Variational Principle for Gravity with Null and Non-null boundaries: A
  Unified Boundary Counter-term   It is common knowledge that the Einstein-Hilbert action does not furnish a
well-posed variational principle. The usual solution to this problem is to add
an extra boundary term to the action, called a counter-term, so that the
variational principle becomes well-posed. When the boundary is spacelike or
timelike, the Gibbons-Hawking-York counter-term is the most widely used. For
null boundaries, we had proposed a counter-term in a previous paper. In this
paper, we extend the previous analysis and propose a counter-term that can be
used to eliminate variations of the "off-the-surface" derivatives of the metric
on any boundary, regardless of its spacelike, timelike or null nature.

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The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
4
The categories of this paper are: cond-mat
22
The categories of this paper are: chao-dyn
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The categories of this paper are: nlin
26
The categories of this paper are: bayes-an
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The categories of this paper are: comp-gas
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-ex
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The categories of this paper are: cs
3
The categories of this paper are: physics
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The categories of this paper are: solv-int
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The categories of this paper are: stat
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-lat
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The categories of this paper are: math