Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On geometry of congruences of null strings in 4-dimensional complex and
  real pseudo-Riemannian spaces   4-dimensional spaces equipped with 2-dimensional (complex holomorphic or real
smooth) completely integrable distributions are considered. The integral
manifolds of such distributions are totally null and totally geodesics
2-dimensional surfaces which are called the null strings. Properties of
congruences (foliations) of such 2-surfaces are studied. Some relations between
properties of congruences of null strings, Petrov-Penrose type of SD Weyl
spinor and algebraic types of the traceless Ricci tensor are analyzed.

23
The categories of this paper are: solv-int
16
The categories of this paper are: stat
29
The categories of this paper are: funct-an
17
The categories of this paper are: q-fin
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
3
The categories of this paper are: physics
24
The categories of this paper are: dg-ga
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
11
The categories of this paper are: hep-lat