In synthetic topology done as a branch of constructive mathematics, a dominance is a set that functions as an analogue of the Sierpinski space. In particular, it allows us to define synthetically a notion of open subset: is a subset of the set of truth values , and a subset of a set is called “open” if its classifying map lands in .
The name dominance is meant to evoke that the set is used to define the domains of a class of partial functions. I.e., in synthetic topology the partial functions whose domain is an open set and in synthetic computability theory the domains of partial computable functions.
Let be the set of truth values. A dominance is a subset such that
The second condition implies that is closed under binary meets , and hence is a sub-meet-semilattice of . In type-theoretic language, the second condition says that is closed under dependent sums.
The elements of are called open truth values.
In homotopy type theory, every univalent type of propositions is a subtype of the type of all propositions . A dominance is a univalent type of propositions with an element such that
We define a subset of an arbitrary set to be open if for each , the proposition “” is an open truth value. The second condition above is equivalent to saying that if is open and also is open, then is open.
Note that for any function , the preimage of any open set is open, since . Thus, any function is “continuous” with respect to this “intrinsic topology.”
It is hard to get very far without an additional assumption that is closed under some joins as well. However, if it were closed under all joins, then it would be all of , since any is the join .
Given a dominance , we say that a set is overt if is closed under -indexed joins. (This is related to, but not identical to, the notion of overt space.) In general it is reasonable to expect discrete sets to be overt in this sense. In some frameworks such as spatial type theory there is a formal notion of “discrete” and we can actually assert that all discrete sets are overt. Otherwise we can assume that specific sets that we expect to be discrete are overt. For instance, we might assume that:
The singleton is a dominance, for which only singletons are overt.
The boolean domain is a dominance. This is the smallest dominance such that the empty set is overt. (In classical mathematics, of course, this and the previous example are the only two dominances, and the theory trivializes.)
The initial -frame is a dominance: the unique -frame homomorphism from to the frame of truth values is an injection, meaning that is a (structural) subset of . This is the smallest dominance such that the natural numbers is overt.
The set of all truth values of the form for some function is often a dominance, though this may not be provable without further assumptions. For instance, this is the case if we assume countable choice or (perhaps) the propositional axiom of choice. When it is a dominance, this is the smallest dominance such that is overt; it is called the Rosolini dominance. Equivalently, it is the set of truth values of the form for some Cauchy real number .
The set of all truth values of the form for some Dedekind real number is also often a dominance, though this also may not be provable without further assumptions.
Pino Rosolini, Continuity and Effectiveness in Topoi, (PhD thesis, 1986), University of Oxford, (pdf)
Martin Escardo, Topology via higher-order intuitionistic logic., unfinished paper, pdf
Martin Escardo, Synthetic topology of data types and classical spaces. Electronic Notes in Theoretical Computer Science, 87:21–156, 2004. pdf
Andrej Bauer and Davorin Lesnik, Metric Spaces in Synthetic Topology, pdf
Last revised on September 12, 2024 at 10:14:16. See the history of this page for a list of all contributions to it.