Prove that the topological closure of a set is definable if the set is definable
$begingroup$
Let $L$ be the language $L={<,=,+,-,cdot, 0,1}$, with standard interpretations, and let $mathcal{A}=langlemathbb{R}, <,=,+,-,cdot,0,1rangle$. Let $Ssubseteqmathbb{R}^n$. Show that if $S$ is definable, then the topological closure of $S$, given as $$bar{S}:={ainmathbb{R}^n:text{every open ball centered at $a$, contains a point of $S$}},$$ is also definable.
My attempt at a solution:
Clearly, the set $bar{S}$ is the set of all points in $S$ as well as the boundary of $S$. So really, what we want is the union between the set $S$, and the set $S'$ which I use to denote the set of all boundary points of $S$. We already have that $S$ is definable by some $L$-formula in the given structure, say $phi^{mathcal{A}}$. It remains to show that the boundary $S'$ is also definable.
The boundary is the set of all points $ainmathbb{R}^n$, so that the following holds:
- $anotin S$
$forallepsilon>0exists sin S$ such that $d(a,s)<epsilon$, where $d$ is the distance function between two points
Let's call the set of elements satisfying the latter property $B$. Clearly the set we are seeking is, $$phi^{mathcal{A}}cup(mathbb{R}^nsetminusphi^{mathcal{A}}cap B).$$ It remains to show that $B$ is definable then. This is where I'm having some trouble. I can't see how to express this in my given language.
logic first-order-logic predicate-logic model-theory
$endgroup$
|
show 5 more comments
$begingroup$
Let $L$ be the language $L={<,=,+,-,cdot, 0,1}$, with standard interpretations, and let $mathcal{A}=langlemathbb{R}, <,=,+,-,cdot,0,1rangle$. Let $Ssubseteqmathbb{R}^n$. Show that if $S$ is definable, then the topological closure of $S$, given as $$bar{S}:={ainmathbb{R}^n:text{every open ball centered at $a$, contains a point of $S$}},$$ is also definable.
My attempt at a solution:
Clearly, the set $bar{S}$ is the set of all points in $S$ as well as the boundary of $S$. So really, what we want is the union between the set $S$, and the set $S'$ which I use to denote the set of all boundary points of $S$. We already have that $S$ is definable by some $L$-formula in the given structure, say $phi^{mathcal{A}}$. It remains to show that the boundary $S'$ is also definable.
The boundary is the set of all points $ainmathbb{R}^n$, so that the following holds:
- $anotin S$
$forallepsilon>0exists sin S$ such that $d(a,s)<epsilon$, where $d$ is the distance function between two points
Let's call the set of elements satisfying the latter property $B$. Clearly the set we are seeking is, $$phi^{mathcal{A}}cup(mathbb{R}^nsetminusphi^{mathcal{A}}cap B).$$ It remains to show that $B$ is definable then. This is where I'm having some trouble. I can't see how to express this in my given language.
logic first-order-logic predicate-logic model-theory
$endgroup$
1
$begingroup$
What difficulty do you have defining $B$? Which part of it do you not know how to express?
$endgroup$
– Eric Wofsey
Dec 5 '18 at 20:54
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I think I figured it out. I think the $psi$ which defines $B$ is given by $$psi:=exists x_1...exists x_n(phiwedgeforallepsilon((0<epsilon)to((x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<epsiloncdotepsilon))).$$ Still, I'm not too sure that is works. Basically I'm trying to say that there is some $sin S$ (i.e. satisfying $phi$), and that in addition to this, the distance between the two is less than epsilon for all epsilon.
$endgroup$
– quanticbolt
Dec 5 '18 at 21:04
$begingroup$
You are on the right lines in your comment using the dot product to express the metric, but your quantification is wrong: informally, $mathbf{y} in S cup S'$ iff for any $epsilon > 0$, there is a $mathbb{x} in S$, with $d(mathbb{x}, mathbb{y}) < epsilon$. Now translate that into logical notation (the $forall$ should come outside the $exists$, not inside).
$endgroup$
– Rob Arthan
Dec 5 '18 at 21:12
$begingroup$
So can I say: $$forallepsilon((0<epsilon)wedgeexists x_1...exists x_n(phiwedge(x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<(epsiloncdotepsilon)))$$
$endgroup$
– quanticbolt
Dec 5 '18 at 21:15
1
$begingroup$
@quanticbolt What if $epsilon = 0$?
$endgroup$
– Alex Kruckman
Dec 6 '18 at 2:18
|
show 5 more comments
$begingroup$
Let $L$ be the language $L={<,=,+,-,cdot, 0,1}$, with standard interpretations, and let $mathcal{A}=langlemathbb{R}, <,=,+,-,cdot,0,1rangle$. Let $Ssubseteqmathbb{R}^n$. Show that if $S$ is definable, then the topological closure of $S$, given as $$bar{S}:={ainmathbb{R}^n:text{every open ball centered at $a$, contains a point of $S$}},$$ is also definable.
My attempt at a solution:
Clearly, the set $bar{S}$ is the set of all points in $S$ as well as the boundary of $S$. So really, what we want is the union between the set $S$, and the set $S'$ which I use to denote the set of all boundary points of $S$. We already have that $S$ is definable by some $L$-formula in the given structure, say $phi^{mathcal{A}}$. It remains to show that the boundary $S'$ is also definable.
The boundary is the set of all points $ainmathbb{R}^n$, so that the following holds:
- $anotin S$
$forallepsilon>0exists sin S$ such that $d(a,s)<epsilon$, where $d$ is the distance function between two points
Let's call the set of elements satisfying the latter property $B$. Clearly the set we are seeking is, $$phi^{mathcal{A}}cup(mathbb{R}^nsetminusphi^{mathcal{A}}cap B).$$ It remains to show that $B$ is definable then. This is where I'm having some trouble. I can't see how to express this in my given language.
logic first-order-logic predicate-logic model-theory
$endgroup$
Let $L$ be the language $L={<,=,+,-,cdot, 0,1}$, with standard interpretations, and let $mathcal{A}=langlemathbb{R}, <,=,+,-,cdot,0,1rangle$. Let $Ssubseteqmathbb{R}^n$. Show that if $S$ is definable, then the topological closure of $S$, given as $$bar{S}:={ainmathbb{R}^n:text{every open ball centered at $a$, contains a point of $S$}},$$ is also definable.
My attempt at a solution:
Clearly, the set $bar{S}$ is the set of all points in $S$ as well as the boundary of $S$. So really, what we want is the union between the set $S$, and the set $S'$ which I use to denote the set of all boundary points of $S$. We already have that $S$ is definable by some $L$-formula in the given structure, say $phi^{mathcal{A}}$. It remains to show that the boundary $S'$ is also definable.
The boundary is the set of all points $ainmathbb{R}^n$, so that the following holds:
- $anotin S$
$forallepsilon>0exists sin S$ such that $d(a,s)<epsilon$, where $d$ is the distance function between two points
Let's call the set of elements satisfying the latter property $B$. Clearly the set we are seeking is, $$phi^{mathcal{A}}cup(mathbb{R}^nsetminusphi^{mathcal{A}}cap B).$$ It remains to show that $B$ is definable then. This is where I'm having some trouble. I can't see how to express this in my given language.
logic first-order-logic predicate-logic model-theory
logic first-order-logic predicate-logic model-theory
edited Dec 5 '18 at 19:57
Asaf Karagila♦
305k33436766
305k33436766
asked Dec 5 '18 at 19:11
quanticboltquanticbolt
769512
769512
1
$begingroup$
What difficulty do you have defining $B$? Which part of it do you not know how to express?
$endgroup$
– Eric Wofsey
Dec 5 '18 at 20:54
$begingroup$
I think I figured it out. I think the $psi$ which defines $B$ is given by $$psi:=exists x_1...exists x_n(phiwedgeforallepsilon((0<epsilon)to((x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<epsiloncdotepsilon))).$$ Still, I'm not too sure that is works. Basically I'm trying to say that there is some $sin S$ (i.e. satisfying $phi$), and that in addition to this, the distance between the two is less than epsilon for all epsilon.
$endgroup$
– quanticbolt
Dec 5 '18 at 21:04
$begingroup$
You are on the right lines in your comment using the dot product to express the metric, but your quantification is wrong: informally, $mathbf{y} in S cup S'$ iff for any $epsilon > 0$, there is a $mathbb{x} in S$, with $d(mathbb{x}, mathbb{y}) < epsilon$. Now translate that into logical notation (the $forall$ should come outside the $exists$, not inside).
$endgroup$
– Rob Arthan
Dec 5 '18 at 21:12
$begingroup$
So can I say: $$forallepsilon((0<epsilon)wedgeexists x_1...exists x_n(phiwedge(x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<(epsiloncdotepsilon)))$$
$endgroup$
– quanticbolt
Dec 5 '18 at 21:15
1
$begingroup$
@quanticbolt What if $epsilon = 0$?
$endgroup$
– Alex Kruckman
Dec 6 '18 at 2:18
|
show 5 more comments
1
$begingroup$
What difficulty do you have defining $B$? Which part of it do you not know how to express?
$endgroup$
– Eric Wofsey
Dec 5 '18 at 20:54
$begingroup$
I think I figured it out. I think the $psi$ which defines $B$ is given by $$psi:=exists x_1...exists x_n(phiwedgeforallepsilon((0<epsilon)to((x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<epsiloncdotepsilon))).$$ Still, I'm not too sure that is works. Basically I'm trying to say that there is some $sin S$ (i.e. satisfying $phi$), and that in addition to this, the distance between the two is less than epsilon for all epsilon.
$endgroup$
– quanticbolt
Dec 5 '18 at 21:04
$begingroup$
You are on the right lines in your comment using the dot product to express the metric, but your quantification is wrong: informally, $mathbf{y} in S cup S'$ iff for any $epsilon > 0$, there is a $mathbb{x} in S$, with $d(mathbb{x}, mathbb{y}) < epsilon$. Now translate that into logical notation (the $forall$ should come outside the $exists$, not inside).
$endgroup$
– Rob Arthan
Dec 5 '18 at 21:12
$begingroup$
So can I say: $$forallepsilon((0<epsilon)wedgeexists x_1...exists x_n(phiwedge(x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<(epsiloncdotepsilon)))$$
$endgroup$
– quanticbolt
Dec 5 '18 at 21:15
1
$begingroup$
@quanticbolt What if $epsilon = 0$?
$endgroup$
– Alex Kruckman
Dec 6 '18 at 2:18
1
1
$begingroup$
What difficulty do you have defining $B$? Which part of it do you not know how to express?
$endgroup$
– Eric Wofsey
Dec 5 '18 at 20:54
$begingroup$
What difficulty do you have defining $B$? Which part of it do you not know how to express?
$endgroup$
– Eric Wofsey
Dec 5 '18 at 20:54
$begingroup$
I think I figured it out. I think the $psi$ which defines $B$ is given by $$psi:=exists x_1...exists x_n(phiwedgeforallepsilon((0<epsilon)to((x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<epsiloncdotepsilon))).$$ Still, I'm not too sure that is works. Basically I'm trying to say that there is some $sin S$ (i.e. satisfying $phi$), and that in addition to this, the distance between the two is less than epsilon for all epsilon.
$endgroup$
– quanticbolt
Dec 5 '18 at 21:04
$begingroup$
I think I figured it out. I think the $psi$ which defines $B$ is given by $$psi:=exists x_1...exists x_n(phiwedgeforallepsilon((0<epsilon)to((x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<epsiloncdotepsilon))).$$ Still, I'm not too sure that is works. Basically I'm trying to say that there is some $sin S$ (i.e. satisfying $phi$), and that in addition to this, the distance between the two is less than epsilon for all epsilon.
$endgroup$
– quanticbolt
Dec 5 '18 at 21:04
$begingroup$
You are on the right lines in your comment using the dot product to express the metric, but your quantification is wrong: informally, $mathbf{y} in S cup S'$ iff for any $epsilon > 0$, there is a $mathbb{x} in S$, with $d(mathbb{x}, mathbb{y}) < epsilon$. Now translate that into logical notation (the $forall$ should come outside the $exists$, not inside).
$endgroup$
– Rob Arthan
Dec 5 '18 at 21:12
$begingroup$
You are on the right lines in your comment using the dot product to express the metric, but your quantification is wrong: informally, $mathbf{y} in S cup S'$ iff for any $epsilon > 0$, there is a $mathbb{x} in S$, with $d(mathbb{x}, mathbb{y}) < epsilon$. Now translate that into logical notation (the $forall$ should come outside the $exists$, not inside).
$endgroup$
– Rob Arthan
Dec 5 '18 at 21:12
$begingroup$
So can I say: $$forallepsilon((0<epsilon)wedgeexists x_1...exists x_n(phiwedge(x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<(epsiloncdotepsilon)))$$
$endgroup$
– quanticbolt
Dec 5 '18 at 21:15
$begingroup$
So can I say: $$forallepsilon((0<epsilon)wedgeexists x_1...exists x_n(phiwedge(x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<(epsiloncdotepsilon)))$$
$endgroup$
– quanticbolt
Dec 5 '18 at 21:15
1
1
$begingroup$
@quanticbolt What if $epsilon = 0$?
$endgroup$
– Alex Kruckman
Dec 6 '18 at 2:18
$begingroup$
@quanticbolt What if $epsilon = 0$?
$endgroup$
– Alex Kruckman
Dec 6 '18 at 2:18
|
show 5 more comments
1 Answer
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$begingroup$
As you wrote in the question, the closure $overline{S}$ is definable by $(forall varepsilon > 0) (exists yin S), d(x,y)<varepsilon$. Expanding this into a first-order formula, given that $S$ is definable by $varphi(x_1,dots,x_n)$, we have:
$$forall varepsilon, ((varepsilon > 0) rightarrow exists y_1dots exists y_n, (varphi(y_1,dots,y_n)land ((x_1-y_1)cdot (x_1-y_1) + dots + (x_n-y_n)cdot (x_n-y_n) < varepsilon cdot varepsilon))).$$
You were on the right track in the comments, but you weren't expressing the bounded quantifiers correctly.
$endgroup$
add a comment |
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$begingroup$
As you wrote in the question, the closure $overline{S}$ is definable by $(forall varepsilon > 0) (exists yin S), d(x,y)<varepsilon$. Expanding this into a first-order formula, given that $S$ is definable by $varphi(x_1,dots,x_n)$, we have:
$$forall varepsilon, ((varepsilon > 0) rightarrow exists y_1dots exists y_n, (varphi(y_1,dots,y_n)land ((x_1-y_1)cdot (x_1-y_1) + dots + (x_n-y_n)cdot (x_n-y_n) < varepsilon cdot varepsilon))).$$
You were on the right track in the comments, but you weren't expressing the bounded quantifiers correctly.
$endgroup$
add a comment |
$begingroup$
As you wrote in the question, the closure $overline{S}$ is definable by $(forall varepsilon > 0) (exists yin S), d(x,y)<varepsilon$. Expanding this into a first-order formula, given that $S$ is definable by $varphi(x_1,dots,x_n)$, we have:
$$forall varepsilon, ((varepsilon > 0) rightarrow exists y_1dots exists y_n, (varphi(y_1,dots,y_n)land ((x_1-y_1)cdot (x_1-y_1) + dots + (x_n-y_n)cdot (x_n-y_n) < varepsilon cdot varepsilon))).$$
You were on the right track in the comments, but you weren't expressing the bounded quantifiers correctly.
$endgroup$
add a comment |
$begingroup$
As you wrote in the question, the closure $overline{S}$ is definable by $(forall varepsilon > 0) (exists yin S), d(x,y)<varepsilon$. Expanding this into a first-order formula, given that $S$ is definable by $varphi(x_1,dots,x_n)$, we have:
$$forall varepsilon, ((varepsilon > 0) rightarrow exists y_1dots exists y_n, (varphi(y_1,dots,y_n)land ((x_1-y_1)cdot (x_1-y_1) + dots + (x_n-y_n)cdot (x_n-y_n) < varepsilon cdot varepsilon))).$$
You were on the right track in the comments, but you weren't expressing the bounded quantifiers correctly.
$endgroup$
As you wrote in the question, the closure $overline{S}$ is definable by $(forall varepsilon > 0) (exists yin S), d(x,y)<varepsilon$. Expanding this into a first-order formula, given that $S$ is definable by $varphi(x_1,dots,x_n)$, we have:
$$forall varepsilon, ((varepsilon > 0) rightarrow exists y_1dots exists y_n, (varphi(y_1,dots,y_n)land ((x_1-y_1)cdot (x_1-y_1) + dots + (x_n-y_n)cdot (x_n-y_n) < varepsilon cdot varepsilon))).$$
You were on the right track in the comments, but you weren't expressing the bounded quantifiers correctly.
answered Dec 6 '18 at 1:42
Alex KruckmanAlex Kruckman
27.6k32658
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add a comment |
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1
$begingroup$
What difficulty do you have defining $B$? Which part of it do you not know how to express?
$endgroup$
– Eric Wofsey
Dec 5 '18 at 20:54
$begingroup$
I think I figured it out. I think the $psi$ which defines $B$ is given by $$psi:=exists x_1...exists x_n(phiwedgeforallepsilon((0<epsilon)to((x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<epsiloncdotepsilon))).$$ Still, I'm not too sure that is works. Basically I'm trying to say that there is some $sin S$ (i.e. satisfying $phi$), and that in addition to this, the distance between the two is less than epsilon for all epsilon.
$endgroup$
– quanticbolt
Dec 5 '18 at 21:04
$begingroup$
You are on the right lines in your comment using the dot product to express the metric, but your quantification is wrong: informally, $mathbf{y} in S cup S'$ iff for any $epsilon > 0$, there is a $mathbb{x} in S$, with $d(mathbb{x}, mathbb{y}) < epsilon$. Now translate that into logical notation (the $forall$ should come outside the $exists$, not inside).
$endgroup$
– Rob Arthan
Dec 5 '18 at 21:12
$begingroup$
So can I say: $$forallepsilon((0<epsilon)wedgeexists x_1...exists x_n(phiwedge(x_1-y_1)cdot(x_1-y_1)+...+(x_n-y_n)cdot(x_n-y_n)<(epsiloncdotepsilon)))$$
$endgroup$
– quanticbolt
Dec 5 '18 at 21:15
1
$begingroup$
@quanticbolt What if $epsilon = 0$?
$endgroup$
– Alex Kruckman
Dec 6 '18 at 2:18