Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Hidden BL Lacertae Objects Near and Far   I describe two difficulties with current research into the cosmic evolution
of BL Lacertae Objects: (1). Possible sample incompleteness due to unrecognized
(i.e., ``hidden'') BL Lacs; and (2). The absence of a viable physical model of
the evolution, to which current and future observations can be compared.

22
The categories of this paper are: chao-dyn
17
The categories of this paper are: q-fin
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
13
The categories of this paper are: quant-ph
19
The categories of this paper are: eess
24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th
30
The categories of this paper are: q-alg
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
3
The categories of this paper are: physics
1
The categories of this paper are: cs
4
The categories of this paper are: cond-mat
5
The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con