Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Neutral bi-Hermitian Gray surfaces   The aim of this paper is to give examples of compact neutral 4-manifolds
$(M,g)$ whose Ricci tensor $\rho$ satisfies the relation $\nabla_X\rho(X,X)
=\frac13X\tau g(X,X)$. We present also a family of new Einstein bi-Hermitian
neutral metrics on ruled surfaces of genus $g>1$.

5
The categories of this paper are: gr-qc
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
13
The categories of this paper are: quant-ph
0
The categories of this paper are: hep-ph
15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
6
The categories of this paper are: astro-ph
29
The categories of this paper are: funct-an
25
The categories of this paper are: patt-sol
37
The categories of this paper are: supr-con
16
The categories of this paper are: stat
7
The categories of this paper are: hep-th
27
The categories of this paper are: comp-gas
20
The categories of this paper are: acc-phys
17
The categories of this paper are: q-fin