In mathematical logic, a theory (also called a formal theory) is a set of sentences in a formal language. Usually a deductive system is understood from context. An element ϕ ∈ T of a theory T is then called an axiom of the theory, and any sentence that follows from the axioms ( T ⊢ ϕ ) is called a theorem of the theory. Every axiom is also a theorem. A first-order theory is a set of first-order sentences.
On the other hand,
A scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested, in accordance with the scientific method [...]. Are testable and make falsifiable predictions. They describe the causes of a particular natural phenomenon and are used to explain and predict aspects of the physical universe or specific areas of inquiry
Perturbation Analysis or Perturbation Methods sound reasonable, but why Theory? How is it a theory?
Just a physicist asking mathematicians. Looking forward to interesting insights.