This is a brief article about the notions of preserving, reflecting, and creating limits and,
by duality, colimits. Preservation is relatively intuitive, but the distinction between
reflection and creation is subtle.
Preservation of Limits
A functor, , preserves limits when it takes limiting cones to limiting cones. As
often happens in category theory texts, the notation focuses on the objects. You’ll often
see things like , but implied is that one direction of
this isomorphism is the canonical morphism . To put it yet
another way, in this example we require to satisfy the universal property
of a product with the projections and .
Other than that subtlety, preservation is fairly intuitive.
Reflection of Limits versus Creation of Limits
A functor, , reflects limits when whenever the image of a cone is a limiting cone,
then the original cone was a limiting cone. For products this would mean that if we
had a wedge , and was the product
of and with projections and , then was the product of and
with projections and .
A functor, , creates limits when whenever the image of a diagram has a limit,
then the diagram itself has a limit and preserves the limiting cones. For products
this would mean if and had a product, , then and have
a product and via the canonical morphism.
Creation of limits implies reflection of limits since we can just ignore the apex of the
cone. While creation is more powerful, often reflection is enough in practice as we usually
have a candidate limit, i.e. a cone. Again, this is often not made too explicit.
Example
Consider the posets:
Failure of reflection
Let with and mapping to
where . Reflection fails because maps to a meet but is not itself a meet.
Failure of creation
If we change the source to just , then creation fails because and have a meet
in the image but not in the source. Reflection succeeds, though, because there are no
non-trivial cones in the source, so every cone (trivially) gets mapped to a limit cone.
It’s just that we don’t have any cones with both and in them.
In general, recasting reflection and creation of limits for posets gives us: Let be
a monotonic function. reflects limits if every lower bound that maps to a meet is
already a meet. creates limits if whenever has a meet for , then
already had a meet and sends the meet of to the meet of .