Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Krein's Resolvent Formula for Self-Adjoint Extensions of Symmetric
  Second Order Elliptic Differential Operators   Given a symmetric, semi-bounded, second order elliptic differential operator
on a bounded domain with $C^{1,1}$ boundary, we provide a Krein-type formula
for the resolvent difference between its Friedrichs extension and an arbitrary
self-adjoint one.

6
The categories of this paper are: astro-ph
5
The categories of this paper are: gr-qc
30
The categories of this paper are: q-alg
23
The categories of this paper are: solv-int
9
The categories of this paper are: nlin
17
The categories of this paper are: q-fin
29
The categories of this paper are: funct-an
15
The categories of this paper are: nucl-ex
33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
8
The categories of this paper are: hep-ex
3
The categories of this paper are: physics
4
The categories of this paper are: cond-mat
31
The categories of this paper are: ao-sci
11
The categories of this paper are: hep-lat
18
The categories of this paper are: econ