Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Ground, a String, Two Elastic Springs. Simple Exactly Solvable
  Models of One-Dimensional Scalar Fields with Concentrated Factors   This paper is a basis of a part of my set of lectures, subject:`Formal
methods in solving differential equations and constructing models of physical
phenomena'.
  Addressed, mainly: postgraduates and related readers.
  Content: a discussion of the simple models (I would rather say, toy models)
of the interaction based on equation arrays of the kind: d^2
u(t,x)/dt^2=c^{2}d^2 u(t,x)/dx^2 -4{\gamma_a}c\delta(x-x_a) Q_a(t)
-4{\gamma_b}c\delta(x-x_b) Q_b(t), Q_a(t) = u(t,x_a), Q_b(t) = u(t,x_b).
Central mathematical points: d'Alembert-Kirchhoff-like Formulae, Finite Rank
Perturbations.

15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
3
The categories of this paper are: physics
11
The categories of this paper are: hep-lat
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
13
The categories of this paper are: quant-ph
26
The categories of this paper are: bayes-an
27
The categories of this paper are: comp-gas
23
The categories of this paper are: solv-int
21
The categories of this paper are: adap-org
5
The categories of this paper are: gr-qc
36
The categories of this paper are: cmp-lg
30
The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th