Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Universal structures with forbidden homomorphisms   We relate the existence problem of universal objects to the properties of
corresponding enriched categories (lifts or expansions). In particular,
extending earlier results, we prove that for every (possibly infinite) regular
set F of finite connected structures there exists a (countable)
\omega-categorical universal structure U for the class Forb(F) (of all
countable structures not containing any homomorphic image of a member of F). We
employ a technique known as homogenization: The universal object U is the
shadow (reduct) of an ultrahomogeneous structure U'.
  We also put the results of this paper in the context of homomorphism
dualities and constraint satisfaction problems. This leads to an alternative
proof of the characterization of finite dualities (given by Tardif and
Ne\v{s}et\v{r}il) as well as of the characterization of infinite-finite
dualities for classes of relational trees given by P. L. Erd\H{o}s,
P\'{a}lv\"{o}lgyi, Tardif and Tardos.
  The notion of regular families of structures is motivated by the recent
characterization of infinite-finite dualities for classes of relational forests
(itself related to regular languages). We show how the notion of a regular
family of relational trees can be extended to regular families of relational
structures. This gives a partial characterization of the existence of a
(countable) \omega-categorical universal object for classes Forb(F).

25
The categories of this paper are: patt-sol
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
31
The categories of this paper are: ao-sci
7
The categories of this paper are: hep-th
36
The categories of this paper are: cmp-lg
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc
17
The categories of this paper are: q-fin
33
The categories of this paper are: plasm-ph
22
The categories of this paper are: chao-dyn