Perturbation theory
Some characteristic phenomena are: the derivation of first-order equations by treating the ε quantities as having negligible squares; the possibility of "isolated solutions", in that varying a solution may not be possible, "or" does not bring anything new; and the question of whether the infinitesimal constraints actually 'integrate', so that their solution does provide small variations. In some form these considerations have a history of centuries in mathematics, but also in physics and engineering. For example, in the geometry of numbers a class of results called "isolation theorems" was recognised, with the topological interpretation of an "open orbit" (of a group action) around a given solution. Perturbation theory also looks at deformations, in general of operators.