In mathematics, progress often consists of recognising the same structure in different contexts - so that one method exploiting it has multiple applications. In fact this is a normal way of proceeding; in the absence of recognisable structure (which might though be hidden) problems tend to fall into the combinatorics classification of matters requiring special arguments.
In category theorystructure is discussed implicitly - as opposed to the explicit discussion typical with the many algebraic structures. Starting with a given class of algebraic structure, such as groups, one can build the category in which the objects are groups and the morphisms are group homomorphisms: that is, of structures on one type, and mappings respecting that structure. Starting with a category C given abstractly, the challenge is to infer what structure it is on the objects that the morphisms 'preserve'.
The term structure was much used in connection with the Bourbaki group's approach. There is even a definition. Structure must definitely include topological space as well as the standard abstract algebra notions. Structure in this sense is probably commensurate with the idea of concrete category that can be presented in a definite way - the topological case means that infinitary operations will be needed. Presentation of a category (analogously to presentation of a group) can in fact be approached in a number of ways, the category structure not being (quite) an algebraic structure in its own right.
The term transport of structure is the 'French' way of expressing covariance or equivariance as a constraint: transfer structure by a surjection and then (if there is an existing structure) compare.
Since any group is a one-object category, a special case of the question about what the morphisms preserve is this: how to consider any group G as a symmetry group? That is answered, as best we can by Cayley's theorem. The analogue in category theory is the Yoneda lemma. One concludes that knowledge on the 'structure' is bound up with what we can say about the representable functors on C. Characterisations of them, in interesting cases, were sought in the 1960s, for application in particular in the moduli problems of algebraic geometry; showing in fact that these are very subtle matters.
In such a case, the category-theoretic solution is to work in a different category (called an arrow category) where the objects are now morphisms of the original category (in the example of subgroups of a given group, the objects would be injections of a subgroup into a larger group).
Categorytheory gives us tools for analyzing such functors: we can talk about natural transformations of functors, and in fact we can use these to assemble the category of functors from one category to another into a category, provided certain set-theoretic constraints are met (universes are a tool used to address these set-theoretic difficulties).
The fundamental theorem of Galois theory is that the functor from a subgroup of the Galois group of a field to its fixed field is an equivalence of categories.
Since categorytheory, or more precisely since the theory of the category of categories, is first-order, it cannot, either as a language or foundation, capture the "central dogma of the axiomatic method: that isomorphic structures are mathematically indistinguishable in their essential properties".
It eliminatory role is stifled by an inability to capture the category of categories as an object of mathematics.
We say that categorytheory is the language of mathematical theories and their relations because it allows us to talk about their general structure in terms of "objects" and "functors", wherein such terms are likewise taken as "syntactic assemblages waiting for a structure of the appropriate sort to give them formulas meaning".
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