Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On active drains and causality   The concept of an \emph{active drain} has been used recently to provide an
optical element which manages to perfectly sink incoming electromagnetic
radiation. Here I show that without prior knowledge of the incoming signal, an
element attempting respond as an active localized drain cannot succeed.

27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
6
The categories of this paper are: astro-ph
37
The categories of this paper are: supr-con
0
The categories of this paper are: hep-ph
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
35
The categories of this paper are: mtrl-th
28
The categories of this paper are: alg-geom
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
21
The categories of this paper are: adap-org