In mathematics, two objects, especially systems of axioms or semantics for them, are called cryptomorphic if they are equivalent but not obviously equivalent. In particular, two definitions or axiomatizations of the ''same'' object are "cryptomorphic" if it is not obvious that they define the same object. Examples of cryptomorphic definitions abound in

matroid In combinatorics, a branch of mathematics, a matroid is a structure that abstracts and generalizes the notion of linear independence in vector spaces. There are many equivalent ways to define a matroid axiomatically, the most significant being ...

theory and others can be found elsewhere, e.g., in

group theory In abstract algebra, group theory studies the algebraic structures known as group (mathematics), groups. The concept of a group is central to abstract algebra: other well-known algebraic structures, such as ring (mathematics), rings, field ...

the definition of a group by a single operation of division, which is not obviously equivalent to the usual three "operations" of identity element, inverse, and multiplication. This word is a play on the many morphisms in mathematics, but "cryptomorphism" is only very distantly related to "

isomorphism In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word i ...

", "

homomorphism In algebra, a homomorphism is a structure-preserving map between two algebraic structures of the same type (such as two groups, two rings, or two vector spaces). The word ''homomorphism'' comes from the Ancient Greek language: () meaning "sa ...

", or "morphisms". The equivalence may in a cryptomorphism, if it is not actual identity, be informal, or may be formalized in terms of a bijection or

equivalence of categories In category theory, a branch of abstract mathematics, an equivalence of categories is a relation between two categories that establishes that these categories are "essentially the same". There are numerous examples of categorical equivalences ...

between the mathematical objects defined by the two cryptomorphic axiom systems.

Etymology

The word was coined by Garrett Birkhoff before 1967, for use in the third edition of his book ''Lattice Theory''. Birkhoff did not give it a formal definition, though others working in the field have made some attempts since.

Use in matroid theory

Its informal sense was popularized (and greatly expanded in scope) by Gian-Carlo Rota in the context of

theory: there are dozens of equivalent axiomatic approaches to matroids, but two different systems of axioms often look very different. In his 1997 book ''Indiscrete Thoughts'', Rota describes the situation as follows: {{cquote, Like many another great idea, matroid theory was invented by one of the great American pioneers,

Hassler Whitney Hassler Whitney (March 23, 1907 – May 10, 1989) was an American mathematician. He was one of the founders of singularity theory, and did foundational work in manifolds, embeddings, immersions, characteristic classes, and geometric integrati ...

. His paper, which is still today the best entry to the subject, flagrantly reveals the unique peculiarity of this field, namely, the exceptional variety of cryptomorphic definitions for a matroid, embarrassingly unrelated to each other and exhibiting wholly different mathematical pedigrees. It is as if one were to condense all trends of present day mathematics onto a single finite structure, a feat that anyone would ''a priori'' deem impossible, were it not for the fact that matroids do exist. Though there are many cryptomorphic concepts in mathematics outside of matroid theory and

universal algebra Universal algebra (sometimes called general algebra) is the field of mathematics that studies algebraic structures themselves, not examples ("models") of algebraic structures. For instance, rather than take particular Group (mathematics), groups as ...

, the word has not caught on among mathematicians generally. It is, however, in fairly wide use among researchers in matroid theory.

References

* Birkhoff, G.: ''Lattice Theory'', 3rd edition. American Mathematical Society Colloquium Publications, Vol. XXV. 1967. * Crapo, H. and Rota, G.-C.: ''On the foundations of combinatorial theory: Combinatorial geometries.'' M.I.T. Press, Cambridge, Mass. 1970. * Elkins, James: Chapter ''Cryptomorphs'' in ''Why Are Our Pictures Puzzles?: On the Modern Origins of Pictorial Complexity'', 1999 * Rota, G.-C.: ''Indiscrete Thoughts'', Birkhäuser Boston, Inc., Boston, MA. 1997. * White, N., editor: ''Theory of Matroids'', Encyclopedia of Mathematics and its Applications, 26. Cambridge University Press, Cambridge. 1986. Mathematical terminology Matroid theory

Etymology

Use in matroid theory

See also

References