In a Lawvere theory, one specifies an algebraic theory by giving a small category $T$ with finite products. Then a model of the theory is given by a functor $T \to Set$ that preserves products.
A sketch is a generalization of this idea, where we take a small category $T$, and pick some distinguished limit and colimit cones. Then a model of the sketch is a functor that preserves these limits and colimits.
A sketch is a small category $T$ equipped with a set $L$ of cones and a set $C$ of cocones. A realized sketch is one where all the cones in $L$ are limit cones and all the cocones in $C$ are colimit cocones.
A limit-sketch is a sketch with $C = \emptyset$, while a colimit-sketch is a sketch with $L = \emptyset$.
A model of a sketch in a category $\mathcal{C}$ is a functor $T\to \mathcal{C}$ taking each cone in $L$ to a limit cone and each cocone in $C$ to a colimit cocone. In particular, $T$ is realized if and only if its identity functor is a model. Frequently the notion of model is restricted to the case $\mathcal{C}=Set$.
A category is called sketchable if it is the category of models (in $Set$) of a sketch.
A Lawvere theory is a special case of a (limit-)sketch, where the category is one with a distinguished object $X$ such that all objects are (isomorphic to) powers of $X$, and $C = \emptyset$ and $L$ is the set of all product cones.
The categories of models of sketches are equivalently the accessible categories.
The categories of models of limit-sketches are the locally presentable categories.
From the discussion there we have that
an accessible category is equivalently:
a locally presentable category is equivalently:
We can “break in half” the difference between the two and define
and
An overview of the theory is given in
An extensive treatment of the links between theories, sketches and models can be found in
That not only every sketchable category is accessible but that conversely every accessible category is sketchable is due to
Last revised on November 9, 2017 at 19:07:06. See the history of this page for a list of all contributions to it.