1. Introduction
A compact set
is semicomputable if its complement
can be effectively exhausted by rational open balls. On the other hand, a compact set
is computable if it is semicomputable and we can effectively enumerate all rational open balls which intersect
S. By the mere definition, it is clear that each computable set is semicomputable, while there exist some examples which show that the reverse implication does not hold. For instance, there exists
such that
is a semicomputable set and
itself is not a computable number (therefore,
is not a computable set) [
1]. Here, by the computable number we mean every real number that can be effectively approximated by a rational point for any given precision. Moreover, there exists a nonempty semicomputable subset of
which does not contain any computable point [
2]. The notions of a semicomputable and a computable set can be naturally defined in the setting of computable metric and topological spaces.
Although the implication
is not true, in general, there are certain additional assumptions on
S under which (
1) holds. For example, each semicomputable compact manifold is computable. Therefore, each semicomputable topological circle is computable, but also a Warsaw circle, which is not a manifold, is computable if it is semicomputable. Moreover, if
S is a semicomputable circularly chainable continuum that is not chainable, then
S is computable. Also, the same is true if
S is a continuum chainable from
a to
b, where
a and
b are computable points (i.e.,
is a semicomputable set) [
3,
4,
5].
We say that a topological space A has a computable type if for any computable topological space the following holds: if S is semicomputable in and homeomorphic to A, then S is computable in . For example, the previously mentioned result for compact manifolds now can be stated in the following way: each compact manifold has a computable type.
The fact that there exists such that is semicomputable, but not computable, means that does not have a computable type. Related to this situation, we have the following concept of computable type.
If A is a topological space and B its subspace, then we say that is a topological pair. We say that a topological pair has a computable type if for any computable topological space the following holds: if is an (topological) embedding such that and are semicomputable sets, then is a computable set.
Using this terminology, we can say that has a computable type if S is a continuum chainable from a to b. In particular, has a computable type.
Recently, Amir and Hoyrup examined conditions under which a finite polyhedron has a computable type (see [
6]). They also examined the computable type and strong computable type in [
7].
Some results regarding computable type and (in)computability of semicomputable sets can be found in [
8,
9,
10].
Inspired by the notion of a topological graph, in [
11] the authors have defined the notion of a chainable graph and have proven that the claims stated for topological graphs, hold for chainable graphs, as well. Namely, a topological graph is considered to be any topological space obtained as a union of finitely many arcs such that every two of them which intersect, intersect in exactly one point which is an endpoint of both of them. The definition of chainable graph arcs has been replaced by chainable continua. It is proved in [
12] that
has computable type if
G is a graph and
E is the set of all its endpoints. In [
11] it was proved that the same holds for chainable graphs.
In this paper, we generalize the notion of a chainable graph from [
11] and we define the notion of a generalized graph. While in a chainable graph, every two edges can intersect only in their endpoints, in the definition of a generalized graph we only demand that two edges (which are chainable continua) intersect finitely many points. More precisely, we say that a triple
is a generalized graph if
G is a topological space,
V is a finite set and
is a finite set of pairs
, where
are distinct points and
is a Hausdorff continuum chainable from
a to
b, such that the following holds:
A is the union of
V and all such
K and for all
such that
we have that
is a finite set. We show that generalized graphs are indeed a generalization of chainable graphs. We prove that
has a computable type if
is a generalized graph and
E is the set of all its endpoints.
It should be mentioned that Amir and Hoyrup in [
7] showed that, for the purpose of studying computable type, computable topological spaces are no more general than computable metric spaces, or in fact the Hilbert cube. However, for the techniques that we use in this paper, a computable topological space is a natural ambient space.
2. Preliminaries
In this section, we recall some basic notions, both from computability and topology, needed to state our main result. For background on computable analysis and some recent results we refer the reader to [
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34].
2.1. Computable Functions
Let
,
. A function
is said to be
computable if there are computable (i.e., recursive) functions
such that
for each
. A function
is said to be
computable if there exists a computable function
such that
for each
,
.
For a set
X, let
denote the family of all finite subsets of
X. A function
is called
computable if the set
is computable and if there is a computable function
such that
for each
.
From now on, let , be some fixed computable function whose range is the set of all nonempty finite subsets of .
2.2. Computable Metric Space
Let be a metric space and be a sequence in X such that is dense in . A triple is said to be a computable metric space if the function , , is computable.
For example, is a computable metric space, where d is the Euclidean metric on for , and is some effective enumeration of .
If is a metric space, and , then by we denote the open ball of radius r centered at x and by we denote the corresponding closed ball. If , we will denote by the closure of A in .
Let be a fixed computable metric space. Let and , . We say that is an (open) rational ball in a computable metric space .
Let
and
be fixed computable functions such that the image of
q is the set of all positive rational numbers and such that
. For
we denote
Note that
is the set of all rational balls in
. For
let
Then,
is the set of all finite (nonempty) unions of rational balls.
Now we can state the following definitions:
a closed set in is a computably enumerable (c.e.) set in if the set is a c.e. subset of ;
a compact set in is a semicomputable set in if the set is a c.e. subset of ;
a compact set in is a computable set in if S is both c.e. and semicomputable in .
These definitions do not depend on the choice of the functions q, , and .
2.3. Computable Topological Space
Let be a topological space and let be a sequence in such that the set is a basis for . A triple is called a computable topological space if there exist c.e. subsets such that:
if are such that , then ;
if are such that , then ;
if and are such that , then there is such that and ;
if are such that , then there are such that , and .
If
is a computable metric space, then it can be shown (see [
35]) that
is a computable metric space, where
is the topology induced by
d and
is the sequence of rational open balls defined earlier.
Let be a computable topological space and, for , let .
We define semicomputable, computably enumerable and computable sets in in the same way as in the case of computable metric spaces. As before, these definitions do not depend on the choice of the sequence ).
If is a computable metric space and , then S is c.e./semicomputable/computable in if and only if S is c.e./semicomputable/computable in .
It should be mentioned that if
is a computable topological space, then the topological space
need not be metrizable (see Example 3.2 in [
35]).
Note that is Hausdorff and second countable if is a computable topological space. So, if S is a compact set in , then S, as a subspace of , is a compact Hausdorff second countable space, which implies that S is a normal second countable space, and therefore, it is metrizable. We will use this fact later.
The proofs of the following facts, which will be used frequently in this paper, can be found in [
35].
Theorem 1. Let be a computable topological space. There exist c.e. subsets such that:
- 1.
if are such that , then ;
- 2.
if are such that , then ;
- 3.
if is a finite family of nonempty compact sets in and is a finite subset of , then for each there is such that
- (i)
;
- (ii)
if are such that , then ;
- (iii)
if and are such that , then .
Proposition 1. Let be a computable topological space, let be a semicomputable set in this space and let . Then, the set is semicomputable in .
We say that
is a
computable point in
if
is a c.e. subset of
. The proof of the following proposition can be found in [
12].
Proposition 2. Let be a computable topological space and let . Then, the following holds: Let us recall the notion of computable type. We say that a topological space A has a computable type if the following holds: if S is a semicomputable set in a computable topological space such that S is homeomorphic to A, then S is computable , . Furthermore, we say that a topological pair has a computable type if the following holds: if S and T are semicomputable sets in a computable topological space such that is homeomorphic to , then S is computable in .
2.4. Chainable and Circularly Chainable Hausdorff Continua
Let
X be a set and
be a finite sequence of subsets of
X. We say that
is a
chain in
X if the following holds:
for all
. Also, we say that
is a
circular chain in
X if the following holds:
for all
.
Let and . We say that covers A if , and we say it covers A from a to b if it is also and .
Let be a metric space. A (circular) chain is said to be an -(circular) chain for some , if for each and it is said to be an open (circular) chain if every is open in . In the same way, we define the notion of a compact (circular) chain.
Let be a continuum, i.e., a connected and compact metric space. We say that is a (circularly) chainable continuum if for every there exists an open -(circular) chain in which covers X. For , we say that is a continuum chainable from a to b if for every there exists an open -chain which covers X from a to b.
The proofs of the following facts can be found in [
5].
Proposition 3. Let be a continuum and .
- 1.
is a chainable continuum from a to b if and only if for each there is a compact ϵ-chain in which covers X from a to b.
- 2.
is a (circularly) chainable continuum if and only if for each there is a compact ϵ-(circular) chain in which covers X.
A Hausdorff continuum is a connected and compact Hausdorff topological space.
Let and be families of sets. We say that refines if for each there exists such that .
Let X be a Hausdorff continuum. We say that X is a (circularly) chainable Hausdorff continuum if for each open cover of X there is an open (circular) chain in X which covers X such that refines . We similarly define that a Hausdorff continuum is chainable from a to b.
It follows easily that a metric space
is a (circularly) chainable continuum if and only if topological space
is a (circularly) chainable Hausdorff continuum (where
is the topology induced by
d). Also,
is a continuum chainable from
a to
b if and only if
is a Hausdorff continuum chainable from
a to
b. See Section 3 in [
3].
Remark 1. Let X and Y be topological spaces and let be a homeomorphism. Then is easy to see that X is a (circularly) chainable Hausdorff continuum if and only if Y is a (circularly) chainable Hausdorff continuum. Furthermore, if , then X is a Hausdorff continuum chainable from a to b if and only if Y is a Hausdorff continuum chainable from to .
2.5. Topological and Chainable Graph
Here, we recall the notions of the topological graph (see [
12]) and its generalization, the chainable graph (see [
11]).
Let be a nonempty finite family of (non-degenerate) line segments in , , such that for all , holds: if then , where a is an endpoint of both I and J. Any topological space G homeomorphic to is called a graph.
For the graph
G we say that
is an
endpoint of
G if there exists an open neighborhood
N of
x in
G such that
N is homeomorphic to
by a homeomorphism which maps
x to 0. If
is the family from the definition of
G, then
x is an endpoint of
G if and only if there exists a unique
such that
x is an endpoint of
I (see [
12]). See
Figure 1.
The following result was proved in [
12].
Theorem 2. Let G be a graph and let E be the set of all endpoints of G. Then has computable type.
Let
A be a topological space. Suppose
V is a finite subset of
A and let
be a finite family of pairs
, where
,
, and
K is a subspace of
A such that
K is a Hausdorff continuum chainable from
a to
b and such that
. Suppose
and the following holds:
- (i)
for all , , there exists at most one K such that ;
- (ii)
if and , then ;
- (iii)
if and , then .
Then, we say that the triple
is a
chainable graph. Also, we say that
is an
endpoint of
if there exists a unique
such that
for some
(note that such a
b also has to be unique by (ii)). See
Figure 2.
For example, if K is a Hausdorff continuum chainable from a to b, , then is a chainable graph and a and b are all of its endpoints.
If
G is a topological graph, then it is not hard to see that there exist
and
V such that
is a chainable graph. On the other hand, if
is a chainable graph, then
G need not be a graph. See [
11].
The proof of the following proposition, a generalization of Theorem 2, can be found in [
11].
Theorem 3. If is a chainable graph and B is the set of all its endpoints, then has a computable type.
3. Generalized Graph
In this section, we consider spaces more general than chainable graphs (and graphs), so-called generalized graphs, and we generalize Theorem 3 by showing that an analog version of this theorem holds for generalized graphs, as well.
Suppose
A is a topological space. Let
be a finite subset of
A and let
be a finite family of pairs
where
,
, and
K is a subspace of
A such that
K is a Hausdorff continuum chainable from
a to
b. Suppose
and that the following holds: if
,
and
, then
.
Then the triple is called a generalized graph.
Let be a chainable graph and let . We say that a is an endpoint of if there exists a unique such that for some .
If , we can think of K as an edge of the generalized graph . Then, is an endpoint of if it is an endpoint of only one of its edges. Also, we can say that the main difference between a chainable and a generalized graph is that we let the edges of the generalized graph intersect in more than two, yet finitely many, points.
Note that each chainable graph is a generalized graph.
Example 1. LetLet and . It is known that K is a continuum chainable from a to b. Therefore, the triple is both a chainable and a generalized graph. Now, let . Since K is also a continuum chainable from b to c, triple is a generalized graph but is not a chainable graph since it does not satisfy the property (ii) from the definition of a chainable graph. The previous example shows us one more distinction between chainable and generalized graphs. Namely, if is a generalized graph and K is its edge, there can exist different pairs of endpoints of K which is not the case in a chainable graph.
We have shown that there exists triple which is a generalized graph but not a chainable graph. In the next three examples we show even more: there is such space A for which there are and V such that is a generalized graph but there are no such and V for which is a chainable graph.
Example 2. It is easy to see that there exist and V such that is a generalized graph.
Example 3. Let A be as in Example 2. Assume that there are and V such that is a chainable graph. Then, there are no such that if To prove so, let us assume the opposite, i.e., let be such that . We will distinguish three cases.
Firstly, let . Let . Since continuum K is chainable from a to b, there is an open ϵ-chain in K which covers K from a to b. If is such that , thenwhich contradicts the connectednes of the set on the left side of the equation since and are disjoint by the definition of a chain. Namely, if then would mean thatwhich is a contradiction. Similarly, if , assuming that leads toa contradiction. Finally, if , we similarly obtainif . If , we obtain a contradiction in the same way.
Hence, we are left with the third case when both and contain exactly one of the points a and b which will also leads to a contradiction (and that will prove the claim). Namely, let . Again, since continuum K is chainable from a to b, there is an open ϵ-chain in K which covers K from a to b. Let be such that . Similarly as before we can prove thatwhich then contradicts the connectedness of the set on the left side of the equation. Example 4. Let A be as in Example 2. We are going to show that there are no and V such that is a chainable graph. To prove so, let us assume the opposite. Then, by the definition of the chainable graph, there is such that (otherwise would be an open set in A, which follows easily from the definition of a chainable graph, but it is clear that is not open in A). Since K is connected and for each This allows us to conclude that for each there is such that . Indeed, let us assume that there exists such that for each it holds . Using (4) we conclude that for there isi.e., there is , such thatIf , by (5) we have thatis a contradiction. Also, if , then there is such that and, because of (5), it isi.e., which contradicts the assumption. This leaves us with the last case possible, . Using (5), this means thatNow, using (4) and the same arguments as above (K is infinite since it is connected, Hausdorff and ), we conclude that there is such thatObviously, there is such that , i.e.,Because is an open set in A (which is not hard to conclude from the definition of a chainable graph), there is such thatSincewe have thatwhich, again, contradicts the assumption. Hence, for each there is such that . Now, let be a projection on the first coordinate. A compact and connected is a segment. Since , obviously , and since for each there is such that , there are , such that . Sinceand is a bijection, we conclude thatand, therefore,i.e., Now, we show that in (6), equality does not hold. Indeed, let us assume thatIf , then . Since , we conclude thata contradiction. If , as above, we conclude that , i.e., . Therefore, K is chainable from to , so there is an open – chain in K which covers K from to . If is such that , similarly as in Example 3, we obtain thatwhich is impossible since is connected. Therefore,which means thatBut, we can conclude even more. It is true thatIndeed, if assume the opposite, for example, , then . Since and is a bijection, we obtain thatsoWe have and so , which implies that or . In both cases we obtain a contradiction with the fact that K is chainable from a to b, as in Example 3. Soand we prove that both of these sets are connected. To show that, assume that is not connected. Therefore, there exists a separation of . Without loss of generality, let , so we can writeBoth F and G are closed in , and therefore, they are closed in A (since is closed in A). Since , F and G are closed in K. Similarly, is closed in A since is open in A. Therefore, (8) gives us a separation of K which is connected, a contradiction. So, since and is connected and compact, there exists such thatSimilarly, there exists such thati.e., which is impossible by Example 3. In this section we are going to prove the following result:
Theorem 4. If is a generalized graph and B is the set of all its endpoints, then has computable type.
Remark 2. Using Remark 1, we conclude the following: if is a generalized graph, B the set of all its endpoints, a topological space and a homeomorphism, thenis generalized graph and is the set of all its endpoints. Namely, if are such that , then , and therefore, is a finite set which, together with the fact that , implies that is a finite set. Let
be a computable topological space and let
S and
T be subsets of
X such that
. We say that
S is
computably enumerable (c.e) up to T if there exists a c.e. subset
of
such that for each
the following holds:
It is obvious that if
S is closed and
S is c.e. up to
S, then
S is a c.e. set in
.
Remark 3. Let be a computable topological space and let ,…, and T be subsets of X such that is c.e. up to T for each . Then, it readily follows that …, is c.e. up to T.
Now we are ready to prove Theorem 4. In view of Remark 2 it is enough to prove the following proposition.
Proposition 4. Let be a computable topological space and let S and T be semicomputable sets in this space. Suppose there are and V such that is a generalized graph and T is the set of all its endpoints. Then, S is computable.
Despite the obvious similarities between chainable and generalized graphs, there are more than slightly differences in the proofs of the main results. In the case of the chainable graph, if we take a continuum (i.e., an edge) K chainable from a to b we know that the intersection with an other edge M can occur only in one of the points a and b, let us say in a, and a is also an endpoint of M, so we can use that fact to write , where are compacts such that and . That is not the case if we have a generalized graph and some technical lemmas are needed in order to solve that problem. So, before giving the proof of Proposition 4, we need certain further facts about chainable continua. Specifically, we have to prove that if it is given a chainable continuum K and its arbitrary point c, we can find a nontrivial subcontinuum L of K which contains c and is small enough. More precisely, we need to show the following:
Lemma 1. Let be a continuum chainable from a to b, and let be arbitrary. For each there is a nontrivial continuum such that and
Firstly, we give straightforward proofs of a few lemmas needed to prove the aforementioned one.
Lemma 2. Let be a continuum chainable from a to b. For each there exists an open ϵ-chain in which covers K from a to b such that is also a chain.
Proof.
Since
is a continuum chainable from
a to
b, there exists a compact
-chain
in
which covers
K from
a to
b. Let us define
Since
for disjoint sets
and
, the number
is positive.
For each
, because
is compact, there exist
and
such that
Let us define
Above the defined
are open sets and because of (
10) it is
,
and
and
for all
such that
. In particular,
.
Furthermore, for arbitrary
, there are
such that
and
. Therefore,
i.e.,
for each
.
It is clear that for all
such that
and
,
it is
because otherwise we would have that
which is impossible.
Since, for each
,
we conclude that
for all
such that
. In particular,
. Therefore,
is an appropriate chain. □
Lemma 3. Let be a continuum chainable from a to b, and let be arbitrary. For each there exists an open ϵ-chain in which covers K from a to b such that:
- 1.
is also a chain;
- 2.
there exists a unique such that .
Proof.
Since is a continuum chainable from a to b, by previous lemma, there is an open -chain in which covers K from a to b such that is also a chain. Let be such that . If then and for each , defines an appropriate chain. Similarly, if the chain is defined by for each , and . Finally, if then the desired chain is . □
Lemma 4. Let X be a set and let and be two chains in X such that refines . Furthermore, suppose that and are such that , and . Then, there exists such that and .
Proof.
We have
since
. If
the conclusion follows from Lemma 3.8 in [
5]. Otherwise, if
, the same lemma, only applied on the chain
defined by
for each
, proves the claim. □
In order to prove Lemma 1, one must consider cases when or and when c is neither a nor b. We have the following definition. If and are finite sequences of subsets of a set X, we say that strongly refines if refines , and .
The proof of the following lemma can be found in [
4].
Lemma 5. Let be a compact metric space. Let , where , , be a sequence of chains such that strongly refines and such that for each and for each . LetThen, S is a continuum chainable from a to b, where , . The proof of this one can be found in [
11].
Lemma 6. Let be a continuum chainable from a to b, , . Let be arbitrary. Then, there exist and such that , L is a continuum chainable from a to c and .
Now, we give the proof of one more lemma, the crucial one for the proof of Lemma 1.
Lemma 7. Let be a compact metric space. Let , where , , be a sequence of open chains such that refines , such that for each and for each and such that is also a chain for each .
LetThen, S is nonempty, connected and compact (i.e., a nonempty continuum). Proof.
Let us denote
Because
refines
(and therefore,
),
is a descending sequence of closed and nonempty subsets of
and since
is compact we have
, i.e.,
.
Furthermore, since S is an intersection of closed subsets of , S is also closed in X which, together with the fact that is compact, implies that S is also compact.
Now, we only have to prove that
S is connected. Let us assume the opposite, i.e., let
be a separation of
S. Let
Since
A and
B are compact and disjoint, we have that
. Let
be such that
for each
.
Now, we will define a sequence
, where
is such that
for each
.
Firstly, notice that each element of the chain
intersects at most one of the sets
A and
B. Indeed, let us assume that there exists
such that
and
. Then, for
and
we have that
which is obviously a contradiction. Therefore, for each
, we conclude that
Since
is a separation of
S and because
refines
, it is
so there exist
such that
Because of (
11), we conclude that
Also, (
12) implies
Namely, if we assume the opposite, i.e.,
then
. So, for
and
,
we have that
i.e., we obtain a contradiction.
We now show that there exists
,
such that
Indeed, let us assume the opposite, i.e., that
for each
. Firstly, let us assume that
. If
for each
, then by assumption we conclude that
for each
. Therefore, for some
and
,
we obtain a contradiction similarly as in (
15). Now, if there is
such that
then
is well defined and we can conclude that
. Again as before, for some
and
,
we obtain a contradiction. The same conclusions arise if
.
Suppose and are such that:
;
;
;
.
We now show that there exist such that:
- (a)
- (b)
- (c)
- (d)
- (e)
Namely, by 3 there is
such that
Similarly as before,
so there is
such that
Obviously,
Similarly, using 2., we conclude that there exists
such that
By (
19), (
20) and 4., one can easily conclude that
Let us now assume that
, i.e.,
. There exists
such that
Indeed, if we assume the opposite, then there exists
such that
which together with (
17) and (
18), implies that
, which is a contradiction since
, i.e.,
. Analogously, there is
such that
Because of (
19) and (
20) it is clear that
and
, so by applying Lemma 4 to (
21) and (
22), we see that there is
,
such that
Using similar arguments, we obtain the same conclusion if
, i.e.,
. This concludes the inductive construction of
and
such that (a)–(e) hold.
Now, to summarize, for each
we have found
such that
In that way, we have constructed a descending sequence
of closed subsets of
, so there exists
In particular,
so by (
16)
. On the other hand,
Therefore,
S is connected. □
Finally, we give the proof of Lemma 1.
Proof.
If or , the conclusion arises from Lemma 6.
Assume now that . Firstly, we will construct a sequence of open chains , , , such that for each :
covers K from a to b and for each ;
is also a chain;
refines ;
such that ;
for each and .
By Lemma 3, there is an open
-chain
in
which covers
K from
a to
b such that
is also a chain and that there exists a unique
such that
Obviously,
In order for
to satisfy all the other conditions, it remains to prove that
. Since
and
, it is
so it follows that
, i.e.,
. Similarly, because
so
, i.e.,
Note that condition 5 implies
.
Let us assume that is an open chain in that satisfies properties 1–4. Since is an open cover of , there is a Lebesque number of . Because is a chainable continuum, by Lemma 3 there is an open -chain in which covers K from a to b such that is also a chain and that there is a unique such that . Because for each , refines .
This concludes the recursive construction of the sequence such that 1–5 hold.
Now, for each
, we want to choose
such that
and
Let us assume that
and for each
numbers
are such that (
24) and (
25) hold. Let
and
The number
is well defined because
and
for each
. Namely, if we assume that
, then
. Because of properties 3 and 4 we have
i.e.,
, which is impossible since
and
. Furthermore, let us assume that there exists
such that
. This, together with multiple use of (
25), implies
So, there exists
such that
. By (
23) and 5, it is
and this yields a contradiction since
. Similarly, the number
is also well defined since
and
for each
. Obviously,
.
Now, we are left to prove that .
Let us assume the opposite, i.e., let
be such that for each
we have
. It is clear that
since
. Furthermore, if
, the number
r is greater than the maximum
defined by (
26) which is obviously a contradiction. Similarly, in case
, a contradiction is obtained using (
27). Therefore, we conclude that
. But we can prove even more. Namely, for each
and
Let us assume the opposite of (
28), i.e.,
from which, together with (
25), we infer that
for some
. Because of (
26), it is
for each
. Considering property 3 this implies that
for some
. Therefore, because
, it is
and since (
30) and (
31) imply
, a contradiction is obtained and (
28) is proven. Similarly, one can easily show that (
29) holds, too.
So, we define
and we conclude, using Lemma 7, that
L is a nonempty continuum in
.
Now we shall prove that
L is nontrivial. Let
S be the set of all finite sequences of the form
,
, such that
and
for each
and let for a sequence
,
It is obvious that
S is infinite and because
, either
or
is infinite. Let us define
, if
is infinite and
, otherwise. Obviously,
is then infinite. Note that because of (
23),
. Now, assume that we have defined
such that
is infinite. Since
we put
, if
is infinite and
, otherwise. It is clear that
is infinite. Therefore, we have constructed a sequence
,
. We can conclude that there is
and because
we conclude that
. Clearly,
. Also, for each
, it is
, so it is
for each
. Therefore,
. Hence,
L is nontrivial.
Now, we are only left to prove that
, which is clear since
and
This concludes the proof. □
Now, we are ready to prove Proposition 4.
Proof.
Suppose
, because otherwise the claim is clear. Recall that
S has to be metrizable. Let
d be the metric on
S which induces the topology on
S (the relative topology on
S in
. For each
,
K is a Hausdorff continuum chainable from
a to
b, and therefore, the metric space
is a continuum chainable from
a to
b. Let
Then, by the definition of a generalized graph
Since
and
are compact in
and disjoint, by Theorem 1 there exists
such that
Together with (
32), we have that
which means that
is semicomputable in
. Since it is finite, Proposition 2 implies that
is computable. In particular, it is c.e. so it is (trivially) c.e. up to
S.
In order to prove that S is c.e. in , we want to prove that K is c.e. up to S for each . In that way, S will be a c.e. up to itself as a finite union of such sets, and therefore, a c.e. set. Together with the fact that S is semicomputable (by assumption), S will be computable and the proof will be completed.
So, let
be arbitrary. Then, there are
such that
. For the sake of simplicity, we will denote
Note that each endpoint is a computable point by Proposition 2, since T is semicomputable and finite.
Let us assume that
. Then
a and
b are endpoints of
, so
a and
b are computable points. If
, then
, so
K is semicomputable by the assumption. Hence,
K is computable as a semicomputable chainable continuum with computable endpoints [
3], in particular,
K is c.e. up to
S. But if
, then equality
implies, similarly as before, that there exists
such that
, and therefore,
K is semicomputable. As in the previous case we conclude that
K is c.e. up to
S.
Now, assume that
and let us denote
i.e., let
be the set of all elements of
K such that they are also contained in some
,
. Obviously,
is finite (by the definition of a generalized graph).
Let us first suppose that
. Then, for each
we have
, i.e.,
Since is a finite union of compact sets, it is also compact and obviously disjoint from K so, as before, we have for some . It follows that K is a semicomputable continuum chainable from a to b, where both a and b are computable points (a and b are endpoints of since ). Therefore, K is a computable set and, in particular, c.e. up to S.
Finally, consider the case
Let us enumerate the set
, i.e., let
for
,
, and let
Using the definition of
it follows that for all
such that
we have
Now, we want to prove that the sets
A and
B defined by
and
are disjoint and both compact in
.
Firstly,
B is compact in
as a finite union of compact sets (a closed ball in a compact
S is also compact). To prove that
A is compact, it is clearly enough to prove the following identity:
Since
to prove (
39), we only have to prove the non trivial inclusion.
It is clear that
so in order to prove that
, it suffices to show that
Indeed, let us assume the opposite, i.e.,
. Then, obviously
. Also, there is no
such that
because then there would exist
such that
which would contradict (
41). Hence,
, i.e.,
, which contradicts (
40). So
.
Since (
39) holds,
A is compact in
. It is obvious that
. So, since
A and
B are disjointed and compacted in
, by Theorem 1 there are
such that
and
Hence,
which, together with (
37) and (
38), implies that
Now, we distinguish four cases depending on whether a and/or b are elements of .
Firstly, suppose
. Then, without loss of generality we may assume that
and
. Since
, there is
,
such that
. Similarly, there is
,
such that
. By Lemma 1 there are nontrivial continua
and
such that
and
Because
L and
N are connected and
and
,
is also connected. Furthermore, by (
42)–(
44) we conclude that
Now, choose
,
(which exists since
L is nontrivial). Because of (
36) and (
43) we conclude that
Moreover,
Indeed, if we assume that
then we have
(because
and
). By (
46), we obtain
, i.e.,
which is not true. Combining (
46) and (
47) we obtain that
so, by Theorem 1, there exists
such that
and
Similarly, there exist
,
and
such that
and
Let and be the subsets of from Theorem 1 and let be a recursive function such that for each .
Now, let be such that . Then, . Namely, if we assume the opposite, i.e., , then is finite, and therefore, closed in K. Since it is also open in K and K is connected, we have . Now, because K is finite and Hausdorff it is discrete and this is a contradiction with the fact that K is connected and ().
So, there exist
and
such that
Furthermore, since is a continuum chainable from a to b, there is a compact r-chain in which covers K and such that and .
Without loss of generality (as in the proof of Lemma 3) we may assume that
Let
be such that
. It follows from (
52) and
that
For each
let
be such that
By (
53) we have
Because of the definition of
r, we also conclude that
so
for each
This, in particular, implies that
Sets and G are all compact since is compact.
Note that for each
, by (
36), we have that
Now, for each
, since
it holds, for each
,
and then
Similarly we have that
for
. This, together with (
36) and the fact that
is a chain implies that the sets
F and
G are disjointed.
Using (
57), one can easily show that
By (
48) and (
50) we have that
and
. Therefore, there exist
such that
,
,
,
,
,
and
. Note that, by (
42) and (
59), it holds
.
So, if is such that , then there exist such that:
- (i)
;
- (ii)
;
- (iii)
;
- (iv)
;
- (v)
.
Let be the set of all for which the statements (i)–(v) hold. Since is a semicomputable set in , the set of all such that (i) holds is a c.e set. Now, it follows easily that is a c.e. set. Let be the set of all for which there exist such that . Then, is c.e by the Projection theorem.
On the other hand, suppose . Then, there exist such that (i)–(v) hold. We want to prove that .
Suppose the opposite, i.e.,
. Since
by (iii), we have
and, by (
45), it holds
Clearly
and
are open in
and disjointed. Since
is connected, in order to yield a contradiction, it is enough to show that both
and
intersect
. It is clear that
. By (
51)
, therefore, because of (v), we have
so
. Similarly, by (
49),
and a contradiction is obtained, i.e.,
. We have proven the following:
This means that
K is c.e. up to
S.
Now suppose and . Similarly as before, we conclude that b is a computable point in and we can assume that .
Again, since
, there is
,
such that
. By Lemma 1, there is a nontrivial continuum
such that
Because
L is connected and
,
is also connected. Furthermore, by (
42),
Now, choose
,
. Similarly, as before, we obtain
so by Theorem 1 there exists
such that
and
Suppose
is such that
. Similarly, as before, there exist
and
such that
Furthermore, since
is a continuum chainable from
a to
b, there is a compact
r-chain
in
which covers
K from
a to
b. Let
be such that
. For each
let
be such that
. We have
for each
and
As above, we define
and
Now, the same as earlier, we conclude that the sets
and
G are compact in
and that
Note that, by (
64) and (
42), it holds
.
As before,
F and
G are disjointed and by (
61) we have that
so there exist
such that
,
,
,
,
,
and
So, if is such that , then there exist such that:
- (i)
;
- (ii)
;
- (iii)
;
- (iv)
;
- (v)
.
Let be the set of all for which the statements (i)–(v) hold and let be the set of all for which there exist such that . Since b is a computable point, the set of all such that (v) holds is a c.e set. Now, it follows easily that is a c.e. set and then is also c.e.
On the other hand, suppose
. Then, there exist
such that (i)–(v) hold. We claim that
. Suppose the opposite, i.e.,
. Since
by (iii), then
and, because of (
60),
Clearly
and
are open and disjointed. Since
is connected, in order to yield a contradiction, it is enough to show that both
and
intersect
, which is clear since
,
by (v) and
(otherwise,
would by (iv) mean that
, which contradicts (
62)). Hence,
. To summarize,
and
i.e.,
K is c.e. up to
S.
Now, we are left with the last case: . Obviously so both a and b are computable points.
Suppose
is such that
. Similarly, as before, there exist
and
such that
Furthermore, since
is a continuum chainable from
a to
b, there exists a compact
r-chain
in
which covers
K from
a to
b. Let
be such that
. It follows
In the same way as before, we define the sets
F and
G and conclude that there exist
such that:
- (i)
;
- (ii)
;
- (iii)
;
- (iv)
;
- (v)
.
Let be the set of all for which the statements (i)–(v) hold and let be the set of all for which there exist such that . As before, it follows that is a c.e. set and that is also c.e.
On the other hand, suppose
. Then, there exist
such that (i)–(v) hold. We want to show that
. Suppose
. Since
by (iii), we have
It follows
,
and
, which is impossible since
K is connected. So
. We have proved that
and
i.e.,
K is c.e. up to
S.
We conclude that each is c.e. up to S. As stated before, that is enough to obtain that S is computable in . □
4. Discussion
In this paper, we have shown that the topological pair of a generalized graph and the set of its endpoints has a computable type. A notion of a generalized graph is inspired by a notion of a graph—a union of finitely many arcs such that distinct arcs intersect in at most one endpoint. However, by the definition of a generalized graph, we let its edges intersect in multiple points as long as there are finitely many of them. A question arises naturally—if we allow infinite intersections of edges, can we state an analogous result for such an object. The answer is negative. Namely, there is continuum
in
chainable from
to
which intersects continuum
chainable from
to
in inifinitely many, yet countable, points. Union
is a semicomputable set that is not computable (see Example 6.2 in [
36]). In fact,
and
are arcs.
In [
7] the authors examined the strong computable type. We will now describe the notion of the strong computable type using terminology from recursive function theory.
Let be any class of functions () such that contains initial (basic) functions and is closed to composition, primitive recursion and the -operator. Note that any (total) computable function belong to . We say that is c.e. relative to if or , where is a function such that each component function of f belongs to . If is a computable topological space and S is a closed set in , we say that S is c.e. in relative to if the set is c.e. relative to . A compact set S in is said to be semicomputable in relative to if the set is c.e. relative to . Finally, S is computable in relative to if it is both c.e. and semicomputable relative to .
We say that a topological space A has strong computable type if in any computable topological space for any the following holds: if S is semicomputable in relative to and S is homeomorphic to A, then S is computable in relative to . We say that a topological pair has a strong computable type if in any computable topological space for any the following holds: if S and T are semicomputable in relative to and is homeomorphic to , then S is computable in relative to .
It is straightforward to check that all proofs in this paper also work if, instead of computable functions, we use functions from any fixed class (where is a class with the property described above). We conclude the following: if is a generalized graph and B is the set of all its endpoints, then has strong computable type.
Also, we would like to highlight how this result helps us to better understand the relationship between computability and topology. We know that for a semicomputable set, in order to be a computable one, it suffices to be computably enumerable, but this is usually not easy to check. Sometimes, the topological properties of a semicomputable set are strong enough that they, together with some additional conditions, imply the computability of the whole set. We have proven that in the case of a generalized graph, computable enumerability (and, therefore, the computability) of the set as a whole follows from the mere topological structure of the set and from the computability of the finitely many specific points (endpoints). Other known results on the computable type also show that there is a strong relationship between topology and computability theory.
In the end, let us mention that in a computable metric space, a semicomputable arc (which, in general, is not computable) can always be approximated by a computable subarc with arbitrary given precision (in the sense of the Hausdorff metric). Moreover, a similar statement holds for semicomputable decomposable chainable continua, see [
5]. In view of this, a natural question is as follows: what can be said about approximations of semicomputable generalized graphs by computable generalized graphs? This can be a further direction of investigation regarding the topic of generalized graphs.