Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On irreducible symplectic 4-folds numerically equivalent to $(K3)^{[2]}$   We address the problem of classification of hyper-K\"ahler fourfolds with
$b_2=23$. In particular we prove some special cases of the Conjecture of
O'Grady about hyper-K\"ahler $4$-folds numerically equivalent to the Hilbert
scheme of two points on a $K3$ surface.

19
The categories of this paper are: eess
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
1
The categories of this paper are: cs
5
The categories of this paper are: gr-qc
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
13
The categories of this paper are: quant-ph
31
The categories of this paper are: ao-sci
21
The categories of this paper are: adap-org
37
The categories of this paper are: supr-con
7
The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin