Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the structure of general solution to the equations of shear-free null
  congruences   We demonstrate that the source of a shear-free null congruence is,
generically, a world sheet of a singular string in the complex Minkowski space.
In particular cases the string degenerates into the Newman's point singularity.
We describe also a time-dependent deformation of the Kerr's congruence which
could indicate the unstability of the Kerr (Kerr-Newman) (electro)vacuum
solution. Then we consider a 5D extension of Minkowski space obtained by
complexification of the time coordinate. Singular locus of a shear-free null
congruence on such an extended space-time consists of a (great) number of point
particles (``markeons'') and 2D surfaces -- light-like wave fronts.

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The categories of this paper are: hep-ph
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
12
The categories of this paper are: nucl-th
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
22
The categories of this paper are: chao-dyn
18
The categories of this paper are: econ
1
The categories of this paper are: cs
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
15
The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
26
The categories of this paper are: bayes-an
37
The categories of this paper are: supr-con