Editing Characterization (mathematics) (section)

== Characterizations in higher mathematics ==
Characterizations are particularly important in higher mathematics, where they take up a large volume of theory in typical undergraduate courses. They are commonly known as "necessary and sufficient conditions," or "if-and-only-if statements." Characterizations help put difficult objects into a form where they are easier to study, and many types of objects in mathematics have multiple characterizations. Sometimes, one characterization in particular particular is more readily generalizable to abstract settings than the others, and it is often chosen as a ''definition'' for the generalized concept.

In [[real analysis]], for example, the [[Completeness of the real numbers|completeness]] property of the real numbers has several useful characterisations:<ref>Abbott, Stephen. ''Understanding Analysis''. New York, Springer, 2016.</ref>

* The [[least-upper-bound property]]
* The [[greatest-lower-bound property]]
* The [[Nested intervals|nested interval property]]
* The [[Bolzano–Weierstrass theorem|Bolzano-Weierstrass theorem]]
* The convergence of [[Cauchy sequence|Cauchy sequences]]

A typical real analysis university course would begin with the first of these, the [[least-upper-bound property]], as an axiomatic definition of the reals (sometimes called the "axiom of completeness" in texts), and gradually prove its way to the last, the convergence of Cauchy sequences. The proofs are quite nontrivial. Among these five characterizations, the Cauchy-sequence perspective turns out to be the easiest to generalize, and is chosen as the ''definition'' for the completeness of an abstract [[metric space]]. However, the least-upper-bound property is often the most useful to prove facts about real numbers themselves, such as the [[intermediate value theorem]]. Thus the most useful and most generalizable characterizations are at times different.

Another example of this phenomenon is found in [[physics]]. [[Hamiltonian mechanics]] is a characterization of classical mechanics, being equivalent to Newton’s laws. However, it is much easier to generalize to [[quantum mechanics]] and [[statistical mechanics]], which is its primary virtue. However, it is a different characterization, [[Lagrangian mechanics]], that is often preferred for the study of classical mechanics itself.<ref>John Robert Taylor. ''Classical Mechanics''. Sausalito, Calif., University Science Books, Cop, 2005.</ref>

Since a characterization result is equivalent to the initial definition or axiom(s) of the object, it can be used as an equivalent definition, from which the original definition can be proved as a theorem. This leads to the question of which definition is “best” in a given situation, out of many possible options. There is no absolute answer, but the ones that are chosen by authors of books or papers is often a matter of aesthetic or pedagogical considerations, as well as convention, history, and tradition. For real numbers, the least-upper-bound property may have been chosen on the grounds of being easier to learn than Cauchy sequences.

One of the most important results in [[complex analysis]] is a characterization result, namely the fact that all locally complex-differentiable functions are analytic (equal to their Taylor series).<ref>Brown, James, and Ruel Churchill. ''Complex Variables and Applications''. McGraw-Hill Science/Engineering/Math, 2009.</ref>

Characterisations are very common in [[abstract algebra]], where they often take the form of “structure theorems,” expressing the structure of an object in a simple form. These results are often very difficult to prove. In the theory of matrices, the Jordan Canonical Form is a characterization, or structure theorem, for complex matrices,<ref>Axler, Sheldon. ''Linear Algebra Done Right''. Springer Nature, 28 Oct. 2023.</ref> and the spectral theorem is likewise for symmetric matrices (if real) or Hermitian matrices (if complex). According to the spectral theorem, the real symmetric matrices are precisely the ones that have a basis of perpendicular eigenvectors<ref>Axler, Sheldon. ''Linear Algebra Done Right''. Springer Nature, 28 Oct. 2023.</ref> (called principal axes in physics). In the theory of [[Group (mathematics)|groups]], there is a structure theorem for [[Finite abelian group|finite abelian groups]], that states that every such group is a [[Direct product of groups|direct product]] of [[Cyclic group|cyclic groups]].<ref>David Steven Dummit, and Richard M Foote. ''Abstract Algebra''. Danvers, John Wiley & Sons, 2004.</ref>

As if-and-only-if statements, characterizations are, in a sense, the “strongest” type of mathematical theorem, which is in line with the difficulty of their proofs. Consider a generic mathematical theorem, that A implies B. If B does not imply A, the theorem may be said to be “underpowered”, as the proved statement B is “weaker” than the ingredient A, being not strong enough to prove A on its own. In a characterization, however, B must imply A also – the proved statement is as strong as the ingredient, and it can be no stronger. In a sense, such a result “uses” all of the structure in A in proving B.