Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: A posteriori error estimation for finite element approximations of a
  PDE-constrained optimization problem in fluid dynamics
6
The categories of this paper are: astro-ph
30
The categories of this paper are: q-alg
3
The categories of this paper are: physics
32
The categories of this paper are: atom-ph
16
The categories of this paper are: stat
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
23
The categories of this paper are: solv-int
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
36
The categories of this paper are: cmp-lg
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin
1
The categories of this paper are: cs