How to prove that this relation is antisymmetric?
$begingroup$
Let $F$ be the set of all functions $f : mathbb{R} to mathbb{R}$. A relation $c$ is defined on $F$ by
$f c g$ if and only if $f(x) ≤ g(x)$ for all $x ∈ mathbb{R} $.
Prove that '$c$' is a partial order.
I have proved that the relation is reflexive. Now I must prove that it is antisymmetric (and later transitive).
My working thus far:
Suppose $a, b ∈ F$. We must prove that if $a c b$ and $b c a$ then $a = b$. Now, if $a c b$ and $b c a$ then $f(a) ≤ f(b)$ and $f(b) ≤ f(a)$. Therefore, $f(a) = f(b)$.
Now, how to prove that $a = b$? There is no indication that $f$ is an injective function.
discrete-mathematics relations
$endgroup$
add a comment |
$begingroup$
Let $F$ be the set of all functions $f : mathbb{R} to mathbb{R}$. A relation $c$ is defined on $F$ by
$f c g$ if and only if $f(x) ≤ g(x)$ for all $x ∈ mathbb{R} $.
Prove that '$c$' is a partial order.
I have proved that the relation is reflexive. Now I must prove that it is antisymmetric (and later transitive).
My working thus far:
Suppose $a, b ∈ F$. We must prove that if $a c b$ and $b c a$ then $a = b$. Now, if $a c b$ and $b c a$ then $f(a) ≤ f(b)$ and $f(b) ≤ f(a)$. Therefore, $f(a) = f(b)$.
Now, how to prove that $a = b$? There is no indication that $f$ is an injective function.
discrete-mathematics relations
$endgroup$
$begingroup$
The relation is on the set of functions, not the reals. $f$ is a variable!
$endgroup$
– ancientmathematician
Apr 1 '17 at 13:39
$begingroup$
Oops, my mistake everyone...apologies, and sincere thanks to all who helped :)
$endgroup$
– Programmer
Apr 1 '17 at 13:43
add a comment |
$begingroup$
Let $F$ be the set of all functions $f : mathbb{R} to mathbb{R}$. A relation $c$ is defined on $F$ by
$f c g$ if and only if $f(x) ≤ g(x)$ for all $x ∈ mathbb{R} $.
Prove that '$c$' is a partial order.
I have proved that the relation is reflexive. Now I must prove that it is antisymmetric (and later transitive).
My working thus far:
Suppose $a, b ∈ F$. We must prove that if $a c b$ and $b c a$ then $a = b$. Now, if $a c b$ and $b c a$ then $f(a) ≤ f(b)$ and $f(b) ≤ f(a)$. Therefore, $f(a) = f(b)$.
Now, how to prove that $a = b$? There is no indication that $f$ is an injective function.
discrete-mathematics relations
$endgroup$
Let $F$ be the set of all functions $f : mathbb{R} to mathbb{R}$. A relation $c$ is defined on $F$ by
$f c g$ if and only if $f(x) ≤ g(x)$ for all $x ∈ mathbb{R} $.
Prove that '$c$' is a partial order.
I have proved that the relation is reflexive. Now I must prove that it is antisymmetric (and later transitive).
My working thus far:
Suppose $a, b ∈ F$. We must prove that if $a c b$ and $b c a$ then $a = b$. Now, if $a c b$ and $b c a$ then $f(a) ≤ f(b)$ and $f(b) ≤ f(a)$. Therefore, $f(a) = f(b)$.
Now, how to prove that $a = b$? There is no indication that $f$ is an injective function.
discrete-mathematics relations
discrete-mathematics relations
edited Apr 1 '17 at 14:08
Rafael Wagner
1,8382923
1,8382923
asked Apr 1 '17 at 13:35
ProgrammerProgrammer
409313
409313
$begingroup$
The relation is on the set of functions, not the reals. $f$ is a variable!
$endgroup$
– ancientmathematician
Apr 1 '17 at 13:39
$begingroup$
Oops, my mistake everyone...apologies, and sincere thanks to all who helped :)
$endgroup$
– Programmer
Apr 1 '17 at 13:43
add a comment |
$begingroup$
The relation is on the set of functions, not the reals. $f$ is a variable!
$endgroup$
– ancientmathematician
Apr 1 '17 at 13:39
$begingroup$
Oops, my mistake everyone...apologies, and sincere thanks to all who helped :)
$endgroup$
– Programmer
Apr 1 '17 at 13:43
$begingroup$
The relation is on the set of functions, not the reals. $f$ is a variable!
$endgroup$
– ancientmathematician
Apr 1 '17 at 13:39
$begingroup$
The relation is on the set of functions, not the reals. $f$ is a variable!
$endgroup$
– ancientmathematician
Apr 1 '17 at 13:39
$begingroup$
Oops, my mistake everyone...apologies, and sincere thanks to all who helped :)
$endgroup$
– Programmer
Apr 1 '17 at 13:43
$begingroup$
Oops, my mistake everyone...apologies, and sincere thanks to all who helped :)
$endgroup$
– Programmer
Apr 1 '17 at 13:43
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
I think you are confuse about your own notation. You define the relation as
Let $mathcal{F}(mathbb{R},mathbb{R})$ be the set of all functions $f: mathbb{R}to mathbb{R}$. So define $color{blue}{c}$ as the relation defined on $mathcal{F}(mathbb{R},mathbb{R})$ as, for $f,g in mathcal{F}(mathbb{R},mathbb{R})$ then $f color{blue} c g Leftrightarrow f(x)leq g(x) , forall x in mathbb{R}$.
Now take a look in your proof: You say that $a,b in mathcal{F}(mathbb{R},mathbb{R})$ and then in your proof you use $a,b$ as variables for $f,g$! But your answer is not all wrong. Let's see it
Suppose $a,b in mathcal{F}(mathbb{R},mathbb{R})$. We want to prove that $(a color{blue }c b $ and $b color{blue}c a )$ implies $a = b$. Now what you've done is
- $a color{blue}c b implies color{red}forall x in mathbb{R}, a(x)color{red}leq b(x)$
- $b color{blue}ca implies color{red}forall x in mathbb{R}, b(x)color{red}leq a(x)$
And then $color{red}{(1)text{ and } (2)text{ together}}$ implies that $forall x in mathbb{R}, a(x) = b(x)$ wich implies that $a = b in mathcal{F}(mathbb{R}, mathbb{R})$. In blue is the relation and in red is important things that should be in your argument. Now you should try to answer transitive. But that should follow directly from arguments of inequality as the one above.
$endgroup$
$begingroup$
Hi, thanks very much Rafael. Yes, I just realised I confused the notation! :)
$endgroup$
– Programmer
Apr 1 '17 at 13:59
$begingroup$
0/ ok thats fine 0/
$endgroup$
– Rafael Wagner
Apr 1 '17 at 14:01
add a comment |
$begingroup$
The relation is over the set of functions, $F$, and not on the set of real numbers, $mathbb R$.
So, here, your $a$s and $b$s are elements of $F$, i.e., functions that map from $mathbb R$ to $mathbb R$.
In your case, $a c b$ means $a le b$, where $a$ and $b$ are functions in $F$. It does not mean $f(a) le f(b)$.
Hence, showing $f=g$ is sufficient.
$endgroup$
add a comment |
$begingroup$
Keep in mind that this is a relation on the functions, not the elements. You seem to be getting confused with this difference. If $f prec g$ and $g prec f$ then $f(x) leq g(x)$ and $f(x) geq g(x)$ for all $x in R$. This means $f=g$. No need for injectivity :)
$endgroup$
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
I think you are confuse about your own notation. You define the relation as
Let $mathcal{F}(mathbb{R},mathbb{R})$ be the set of all functions $f: mathbb{R}to mathbb{R}$. So define $color{blue}{c}$ as the relation defined on $mathcal{F}(mathbb{R},mathbb{R})$ as, for $f,g in mathcal{F}(mathbb{R},mathbb{R})$ then $f color{blue} c g Leftrightarrow f(x)leq g(x) , forall x in mathbb{R}$.
Now take a look in your proof: You say that $a,b in mathcal{F}(mathbb{R},mathbb{R})$ and then in your proof you use $a,b$ as variables for $f,g$! But your answer is not all wrong. Let's see it
Suppose $a,b in mathcal{F}(mathbb{R},mathbb{R})$. We want to prove that $(a color{blue }c b $ and $b color{blue}c a )$ implies $a = b$. Now what you've done is
- $a color{blue}c b implies color{red}forall x in mathbb{R}, a(x)color{red}leq b(x)$
- $b color{blue}ca implies color{red}forall x in mathbb{R}, b(x)color{red}leq a(x)$
And then $color{red}{(1)text{ and } (2)text{ together}}$ implies that $forall x in mathbb{R}, a(x) = b(x)$ wich implies that $a = b in mathcal{F}(mathbb{R}, mathbb{R})$. In blue is the relation and in red is important things that should be in your argument. Now you should try to answer transitive. But that should follow directly from arguments of inequality as the one above.
$endgroup$
$begingroup$
Hi, thanks very much Rafael. Yes, I just realised I confused the notation! :)
$endgroup$
– Programmer
Apr 1 '17 at 13:59
$begingroup$
0/ ok thats fine 0/
$endgroup$
– Rafael Wagner
Apr 1 '17 at 14:01
add a comment |
$begingroup$
I think you are confuse about your own notation. You define the relation as
Let $mathcal{F}(mathbb{R},mathbb{R})$ be the set of all functions $f: mathbb{R}to mathbb{R}$. So define $color{blue}{c}$ as the relation defined on $mathcal{F}(mathbb{R},mathbb{R})$ as, for $f,g in mathcal{F}(mathbb{R},mathbb{R})$ then $f color{blue} c g Leftrightarrow f(x)leq g(x) , forall x in mathbb{R}$.
Now take a look in your proof: You say that $a,b in mathcal{F}(mathbb{R},mathbb{R})$ and then in your proof you use $a,b$ as variables for $f,g$! But your answer is not all wrong. Let's see it
Suppose $a,b in mathcal{F}(mathbb{R},mathbb{R})$. We want to prove that $(a color{blue }c b $ and $b color{blue}c a )$ implies $a = b$. Now what you've done is
- $a color{blue}c b implies color{red}forall x in mathbb{R}, a(x)color{red}leq b(x)$
- $b color{blue}ca implies color{red}forall x in mathbb{R}, b(x)color{red}leq a(x)$
And then $color{red}{(1)text{ and } (2)text{ together}}$ implies that $forall x in mathbb{R}, a(x) = b(x)$ wich implies that $a = b in mathcal{F}(mathbb{R}, mathbb{R})$. In blue is the relation and in red is important things that should be in your argument. Now you should try to answer transitive. But that should follow directly from arguments of inequality as the one above.
$endgroup$
$begingroup$
Hi, thanks very much Rafael. Yes, I just realised I confused the notation! :)
$endgroup$
– Programmer
Apr 1 '17 at 13:59
$begingroup$
0/ ok thats fine 0/
$endgroup$
– Rafael Wagner
Apr 1 '17 at 14:01
add a comment |
$begingroup$
I think you are confuse about your own notation. You define the relation as
Let $mathcal{F}(mathbb{R},mathbb{R})$ be the set of all functions $f: mathbb{R}to mathbb{R}$. So define $color{blue}{c}$ as the relation defined on $mathcal{F}(mathbb{R},mathbb{R})$ as, for $f,g in mathcal{F}(mathbb{R},mathbb{R})$ then $f color{blue} c g Leftrightarrow f(x)leq g(x) , forall x in mathbb{R}$.
Now take a look in your proof: You say that $a,b in mathcal{F}(mathbb{R},mathbb{R})$ and then in your proof you use $a,b$ as variables for $f,g$! But your answer is not all wrong. Let's see it
Suppose $a,b in mathcal{F}(mathbb{R},mathbb{R})$. We want to prove that $(a color{blue }c b $ and $b color{blue}c a )$ implies $a = b$. Now what you've done is
- $a color{blue}c b implies color{red}forall x in mathbb{R}, a(x)color{red}leq b(x)$
- $b color{blue}ca implies color{red}forall x in mathbb{R}, b(x)color{red}leq a(x)$
And then $color{red}{(1)text{ and } (2)text{ together}}$ implies that $forall x in mathbb{R}, a(x) = b(x)$ wich implies that $a = b in mathcal{F}(mathbb{R}, mathbb{R})$. In blue is the relation and in red is important things that should be in your argument. Now you should try to answer transitive. But that should follow directly from arguments of inequality as the one above.
$endgroup$
I think you are confuse about your own notation. You define the relation as
Let $mathcal{F}(mathbb{R},mathbb{R})$ be the set of all functions $f: mathbb{R}to mathbb{R}$. So define $color{blue}{c}$ as the relation defined on $mathcal{F}(mathbb{R},mathbb{R})$ as, for $f,g in mathcal{F}(mathbb{R},mathbb{R})$ then $f color{blue} c g Leftrightarrow f(x)leq g(x) , forall x in mathbb{R}$.
Now take a look in your proof: You say that $a,b in mathcal{F}(mathbb{R},mathbb{R})$ and then in your proof you use $a,b$ as variables for $f,g$! But your answer is not all wrong. Let's see it
Suppose $a,b in mathcal{F}(mathbb{R},mathbb{R})$. We want to prove that $(a color{blue }c b $ and $b color{blue}c a )$ implies $a = b$. Now what you've done is
- $a color{blue}c b implies color{red}forall x in mathbb{R}, a(x)color{red}leq b(x)$
- $b color{blue}ca implies color{red}forall x in mathbb{R}, b(x)color{red}leq a(x)$
And then $color{red}{(1)text{ and } (2)text{ together}}$ implies that $forall x in mathbb{R}, a(x) = b(x)$ wich implies that $a = b in mathcal{F}(mathbb{R}, mathbb{R})$. In blue is the relation and in red is important things that should be in your argument. Now you should try to answer transitive. But that should follow directly from arguments of inequality as the one above.
edited Jan 3 at 12:41
answered Apr 1 '17 at 13:57
Rafael WagnerRafael Wagner
1,8382923
1,8382923
$begingroup$
Hi, thanks very much Rafael. Yes, I just realised I confused the notation! :)
$endgroup$
– Programmer
Apr 1 '17 at 13:59
$begingroup$
0/ ok thats fine 0/
$endgroup$
– Rafael Wagner
Apr 1 '17 at 14:01
add a comment |
$begingroup$
Hi, thanks very much Rafael. Yes, I just realised I confused the notation! :)
$endgroup$
– Programmer
Apr 1 '17 at 13:59
$begingroup$
0/ ok thats fine 0/
$endgroup$
– Rafael Wagner
Apr 1 '17 at 14:01
$begingroup$
Hi, thanks very much Rafael. Yes, I just realised I confused the notation! :)
$endgroup$
– Programmer
Apr 1 '17 at 13:59
$begingroup$
Hi, thanks very much Rafael. Yes, I just realised I confused the notation! :)
$endgroup$
– Programmer
Apr 1 '17 at 13:59
$begingroup$
0/ ok thats fine 0/
$endgroup$
– Rafael Wagner
Apr 1 '17 at 14:01
$begingroup$
0/ ok thats fine 0/
$endgroup$
– Rafael Wagner
Apr 1 '17 at 14:01
add a comment |
$begingroup$
The relation is over the set of functions, $F$, and not on the set of real numbers, $mathbb R$.
So, here, your $a$s and $b$s are elements of $F$, i.e., functions that map from $mathbb R$ to $mathbb R$.
In your case, $a c b$ means $a le b$, where $a$ and $b$ are functions in $F$. It does not mean $f(a) le f(b)$.
Hence, showing $f=g$ is sufficient.
$endgroup$
add a comment |
$begingroup$
The relation is over the set of functions, $F$, and not on the set of real numbers, $mathbb R$.
So, here, your $a$s and $b$s are elements of $F$, i.e., functions that map from $mathbb R$ to $mathbb R$.
In your case, $a c b$ means $a le b$, where $a$ and $b$ are functions in $F$. It does not mean $f(a) le f(b)$.
Hence, showing $f=g$ is sufficient.
$endgroup$
add a comment |
$begingroup$
The relation is over the set of functions, $F$, and not on the set of real numbers, $mathbb R$.
So, here, your $a$s and $b$s are elements of $F$, i.e., functions that map from $mathbb R$ to $mathbb R$.
In your case, $a c b$ means $a le b$, where $a$ and $b$ are functions in $F$. It does not mean $f(a) le f(b)$.
Hence, showing $f=g$ is sufficient.
$endgroup$
The relation is over the set of functions, $F$, and not on the set of real numbers, $mathbb R$.
So, here, your $a$s and $b$s are elements of $F$, i.e., functions that map from $mathbb R$ to $mathbb R$.
In your case, $a c b$ means $a le b$, where $a$ and $b$ are functions in $F$. It does not mean $f(a) le f(b)$.
Hence, showing $f=g$ is sufficient.
answered Apr 1 '17 at 13:40
GoodDeedsGoodDeeds
10.3k31435
10.3k31435
add a comment |
add a comment |
$begingroup$
Keep in mind that this is a relation on the functions, not the elements. You seem to be getting confused with this difference. If $f prec g$ and $g prec f$ then $f(x) leq g(x)$ and $f(x) geq g(x)$ for all $x in R$. This means $f=g$. No need for injectivity :)
$endgroup$
add a comment |
$begingroup$
Keep in mind that this is a relation on the functions, not the elements. You seem to be getting confused with this difference. If $f prec g$ and $g prec f$ then $f(x) leq g(x)$ and $f(x) geq g(x)$ for all $x in R$. This means $f=g$. No need for injectivity :)
$endgroup$
add a comment |
$begingroup$
Keep in mind that this is a relation on the functions, not the elements. You seem to be getting confused with this difference. If $f prec g$ and $g prec f$ then $f(x) leq g(x)$ and $f(x) geq g(x)$ for all $x in R$. This means $f=g$. No need for injectivity :)
$endgroup$
Keep in mind that this is a relation on the functions, not the elements. You seem to be getting confused with this difference. If $f prec g$ and $g prec f$ then $f(x) leq g(x)$ and $f(x) geq g(x)$ for all $x in R$. This means $f=g$. No need for injectivity :)
answered Apr 1 '17 at 13:39
phunfdphunfd
29629
29629
add a comment |
add a comment |
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$begingroup$
The relation is on the set of functions, not the reals. $f$ is a variable!
$endgroup$
– ancientmathematician
Apr 1 '17 at 13:39
$begingroup$
Oops, my mistake everyone...apologies, and sincere thanks to all who helped :)
$endgroup$
– Programmer
Apr 1 '17 at 13:43