Motivated by the answer to this question–"What kind of “symmetry” is the symmetric group about?", I read the article about dual graph. It is said in this article that "the term 'dual' is used because this property is symmetric, meaning that if H is a dual of G, then G is a dual of H (if G is connected)." In mathematics, duality is a very important phenomenon and one may immediately come up with lots of examples(e.g., dual space in functional analysis). At the same times, there are many different kinds of symmetry in mathematics. This wiki article points out that "A high-level concept related to symmetry is mathematical duality.".
Here is my question:
What's the difference and relationship between "duality" and "symmetry"?
Best Answer
No, duality and symmetry are not the same thing. Although in many contexts "the dual of" is a symmetric relation, this is not invariably the case (e.g. the dual of the dual of a topological vector space need not be the original).
Moreover symmetry is not just about symmetric relations; it has to do mainly with automorphisms of algebraic, geometric or combinatorial structures. Those structure preserving automorphisms (including trivial the identity mapping) form a group, and we'd refer to it as the symmetry group of the structure.
As you note, there are many kinds of symmetry. Some symmetries have order two but many do not. Indeed the group of symmetries may combine elements that have finite order with those having infinite order, elements that have discrete action with some that are continuous mappings. The symmetries of a right circular cylinder, for example, would include discrete actions like reflection in a midplane as well as continuous actions of rotation about the axis.
If you are looking for a fundamental difference, perhaps it should be noted that duality often involves different categories, i.e. the dual may belong to a different category than the original, while symmetry involves not only the same category but actually a mapping of the same object to itself.