Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spinor algebra and null solutions of the wave equation   In this paper we exploit the ideas and formalisms of twistor theory, to show
how, on Minkowski space, given a null solution of the wave equation, there are
precisely two null directions in $\ker df$, at least one of which is a
shear-free ray congruence.

18
The categories of this paper are: econ
17
The categories of this paper are: q-fin
7
The categories of this paper are: hep-th
37
The categories of this paper are: supr-con
21
The categories of this paper are: adap-org
6
The categories of this paper are: astro-ph
25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
30
The categories of this paper are: q-alg
13
The categories of this paper are: quant-ph
26
The categories of this paper are: bayes-an
8
The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg