Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Higher-Dimensional Algebra IV: 2-Tangles   Just as knots and links can be algebraically described as certain morphisms
in the category of tangles in 3 dimensions, compact surfaces smoothly embedded
in R^4 can be described as certain 2-morphisms in the 2-category of `2-tangles
in 4 dimensions'. Using the work of Carter, Rieger and Saito, we prove that
this 2-category is the `free semistrict braided monoidal 2-category with duals
on one unframed self-dual object'. By this universal property, any unframed
self-dual object in a braided monoidal 2-category with duals determines an
invariant of 2-tangles in 4 dimensions.

8
The categories of this paper are: hep-ex
13
The categories of this paper are: quant-ph
12
The categories of this paper are: nucl-th
10
The categories of this paper are: q-bio
20
The categories of this paper are: acc-phys
33
The categories of this paper are: plasm-ph
4
The categories of this paper are: cond-mat
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
0
The categories of this paper are: hep-ph
5
The categories of this paper are: gr-qc
11
The categories of this paper are: hep-lat
14
The categories of this paper are: math-ph
7
The categories of this paper are: hep-th
24
The categories of this paper are: dg-ga
35
The categories of this paper are: mtrl-th