Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Extensions of the reduced group C*-algebra of a free product of amenable
  groups   We prove that the unitary equivalence classes of extensions of C*_r(G) by any
sigma-unital stable C"-algebra, taken modulo extensions which split via an
asymptotic homomorphism, form a group which can be calculated from the
universal coefficient theorem of KK-theory when G is a free product of a
countable collection of amenable groups.

32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
27
The categories of this paper are: comp-gas
18
The categories of this paper are: econ
24
The categories of this paper are: dg-ga
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
7
The categories of this paper are: hep-th
34
The categories of this paper are: chem-ph
25
The categories of this paper are: patt-sol
15
The categories of this paper are: nucl-ex
1
The categories of this paper are: cs