Categorical Aspects
Cobordisms are objects of study in their own right, apart from cobordism classes. Cobordisms form a category whose objects are closed manifolds and whose morphisms are cobordisms. Roughly speaking, composition is given by gluing together cobordisms end-to-end: the composition of and is defined by gluing the right end of the first to the left end of the second, yielding . A cobordism is a kind of cospan: M → W ← N.
A topological quantum field theory is a monoidal functor from a category of cobordisms to a category of vector spaces. That is, it is a functor whose value on a disjoint union of manifolds is equivalent to the tensor product of its values on each of the constituent manifolds.
In low dimensions, the bordism question is trivial, but the category of cobordism is still interesting. For instance, the disk bounding the circle corresponds to a null-ary operation, while the cylinder corresponds to a 1-ary operation and the pair of pants to a binary operation.
Read more about this topic: Cobordism
Famous quotes containing the words categorical and/or aspects:
“We do the same thing to parents that we do to children. We insist that they are some kind of categorical abstraction because they produced a child. They were people before that, and they’re still people in all other areas of their lives. But when it comes to the state of parenthood they are abruptly heir to a whole collection of virtues and feelings that are assigned to them with a fine arbitrary disregard for individuality.”
—Leontine Young (20th century)
“The happiest two-job marriages I saw during my research were ones in which men and women shared the housework and parenting. What couples called good communication often meant that they were good at saying thanks to one another for small aspects of taking care of the family. Making it to the school play, helping a child read, cooking dinner in good spirit, remembering the grocery list,... these were silver and gold of the marital exchange.”
—Arlie Hochschild (20th century)