Proof that $(a, b) mathrel{R} (c, d)$ iff $ad = bc$ is an equivalence relation
Let $X = {(a,b) mid a,b in Bbb Z; b ne 0}$. We define $(a,b)mathrel R (c,d)$ iff $ad = bc$. Prove that $R$ is an equivalence relation on the set $X$. Which known set do the equivalence classes of the relation form?
Any help on solving this please?
elementary-set-theory relations equivalence-relations
add a comment |
Let $X = {(a,b) mid a,b in Bbb Z; b ne 0}$. We define $(a,b)mathrel R (c,d)$ iff $ad = bc$. Prove that $R$ is an equivalence relation on the set $X$. Which known set do the equivalence classes of the relation form?
Any help on solving this please?
elementary-set-theory relations equivalence-relations
3
Have you tried showing the relation is reflexive, symmetric and transitive?
– user99680
Dec 18 '13 at 17:06
Note that $(a,b)R (c,d)$ iff $$frac{a}{b} = frac{c}{d}$$ Can you now guess what the set of equivalence classes is?
– Prahlad Vaidyanathan
Dec 18 '13 at 17:07
1
Hint:Recall the definition of rational numbers. $mathbb{Q}={frac{a}{b}|a,bin mathbb{Z},bne 0}$.
– Farshad Nahangi
Dec 18 '13 at 17:07
add a comment |
Let $X = {(a,b) mid a,b in Bbb Z; b ne 0}$. We define $(a,b)mathrel R (c,d)$ iff $ad = bc$. Prove that $R$ is an equivalence relation on the set $X$. Which known set do the equivalence classes of the relation form?
Any help on solving this please?
elementary-set-theory relations equivalence-relations
Let $X = {(a,b) mid a,b in Bbb Z; b ne 0}$. We define $(a,b)mathrel R (c,d)$ iff $ad = bc$. Prove that $R$ is an equivalence relation on the set $X$. Which known set do the equivalence classes of the relation form?
Any help on solving this please?
elementary-set-theory relations equivalence-relations
elementary-set-theory relations equivalence-relations
edited Jan 8 '15 at 20:31
6005
35.7k751125
35.7k751125
asked Dec 18 '13 at 17:03
imreimre
135
135
3
Have you tried showing the relation is reflexive, symmetric and transitive?
– user99680
Dec 18 '13 at 17:06
Note that $(a,b)R (c,d)$ iff $$frac{a}{b} = frac{c}{d}$$ Can you now guess what the set of equivalence classes is?
– Prahlad Vaidyanathan
Dec 18 '13 at 17:07
1
Hint:Recall the definition of rational numbers. $mathbb{Q}={frac{a}{b}|a,bin mathbb{Z},bne 0}$.
– Farshad Nahangi
Dec 18 '13 at 17:07
add a comment |
3
Have you tried showing the relation is reflexive, symmetric and transitive?
– user99680
Dec 18 '13 at 17:06
Note that $(a,b)R (c,d)$ iff $$frac{a}{b} = frac{c}{d}$$ Can you now guess what the set of equivalence classes is?
– Prahlad Vaidyanathan
Dec 18 '13 at 17:07
1
Hint:Recall the definition of rational numbers. $mathbb{Q}={frac{a}{b}|a,bin mathbb{Z},bne 0}$.
– Farshad Nahangi
Dec 18 '13 at 17:07
3
3
Have you tried showing the relation is reflexive, symmetric and transitive?
– user99680
Dec 18 '13 at 17:06
Have you tried showing the relation is reflexive, symmetric and transitive?
– user99680
Dec 18 '13 at 17:06
Note that $(a,b)R (c,d)$ iff $$frac{a}{b} = frac{c}{d}$$ Can you now guess what the set of equivalence classes is?
– Prahlad Vaidyanathan
Dec 18 '13 at 17:07
Note that $(a,b)R (c,d)$ iff $$frac{a}{b} = frac{c}{d}$$ Can you now guess what the set of equivalence classes is?
– Prahlad Vaidyanathan
Dec 18 '13 at 17:07
1
1
Hint:Recall the definition of rational numbers. $mathbb{Q}={frac{a}{b}|a,bin mathbb{Z},bne 0}$.
– Farshad Nahangi
Dec 18 '13 at 17:07
Hint:Recall the definition of rational numbers. $mathbb{Q}={frac{a}{b}|a,bin mathbb{Z},bne 0}$.
– Farshad Nahangi
Dec 18 '13 at 17:07
add a comment |
3 Answers
3
active
oldest
votes
You need to show that the given relation is
reflexive
For all $(a, b) in X$, $ab = ba$. This is clearly true. Hence R is reflexive.symmetric
For all $(a, b), (c, d) in X$, suppose $(a, b) R(c, d).$ Then $ad = bc $ if and only if $cb = da$ if and only if $(c, d) R (a,b)$. Therefore, R is
symmetric.transitive
Now, see what you can do with the following: Take arbitrary $(a, b), (c, d), (e, f)$ and assume $(a, b)R (c, d)$, and $(c, d) R (e, f)$. So
$$ad = bc,quad text{and} quad cf = de$$ Now, we use a little algebra to show that this implies $af = be$, and hence $(a, b) R (e, f)$.
$ad = bc iff adf = bcf = b(cf).$ Also, we have $cf = de$. So $adf = b(de) iff af = be$, as desired!
The relation has all three properties, and hence, is by definition, an equivalence relation.
Hint: $$(a, b) R (c, d) ;text{ if and only if };frac{a}{b} = frac{c}{d}; text{ if and only if } ;ad = bc, ;;(bneq 0, dneq 0)$$
nice, thanks a lot!
– imre
Dec 18 '13 at 17:31
You're welcome!
– amWhy
Dec 18 '13 at 17:34
Helped me too. p.s. Crazy reputation.
– Det
Sep 21 '17 at 12:01
add a comment |
To show this is an equivalence relation, you need to show, for pairs $(a,b),(c,d), (e,f)$:
i)$(a,b)R(a,b)$, for any pair $(a,b)$
ii) If $(a,b)R(c,d)$ , then $(c,d)R(a,b)$
iii)If $(a,b)R(c,d)$ and $(c,d)R(e,f)$ , then $(a,b)R(e,f)$.
Can you see how to it?
Let's set up i): We want to show that, for any pair $(a,b)$ , i.e., for any choice of integers $a,b$ , we have $(a,b)R(a,b)$. This means that, according to the definition of $R$, we have:
$(a,b)R(a,b)$ iff $ab=ba$ . Can you do the rest?
What numbers should I pick?
– imre
Dec 18 '13 at 17:13
It is supposed to be true for any choice of numbers. So try assuming that $a,b,c,d,e,f$ are any integers. Can you see how?
– user99680
Dec 18 '13 at 17:15
add a comment |
Consider that $$(a,b)R(c,d)quad text{ iff }quad ad = bcquad text{ iff }quadfrac{a}{b}=frac{c}{d}$$
and since $=$ is an equivalence relation, $R$ is an equivalence one, and $mathbb{Q}$ is an example of such classes.
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f611902%2fproof-that-a-b-mathrelr-c-d-iff-ad-bc-is-an-equivalence-relation%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
You need to show that the given relation is
reflexive
For all $(a, b) in X$, $ab = ba$. This is clearly true. Hence R is reflexive.symmetric
For all $(a, b), (c, d) in X$, suppose $(a, b) R(c, d).$ Then $ad = bc $ if and only if $cb = da$ if and only if $(c, d) R (a,b)$. Therefore, R is
symmetric.transitive
Now, see what you can do with the following: Take arbitrary $(a, b), (c, d), (e, f)$ and assume $(a, b)R (c, d)$, and $(c, d) R (e, f)$. So
$$ad = bc,quad text{and} quad cf = de$$ Now, we use a little algebra to show that this implies $af = be$, and hence $(a, b) R (e, f)$.
$ad = bc iff adf = bcf = b(cf).$ Also, we have $cf = de$. So $adf = b(de) iff af = be$, as desired!
The relation has all three properties, and hence, is by definition, an equivalence relation.
Hint: $$(a, b) R (c, d) ;text{ if and only if };frac{a}{b} = frac{c}{d}; text{ if and only if } ;ad = bc, ;;(bneq 0, dneq 0)$$
nice, thanks a lot!
– imre
Dec 18 '13 at 17:31
You're welcome!
– amWhy
Dec 18 '13 at 17:34
Helped me too. p.s. Crazy reputation.
– Det
Sep 21 '17 at 12:01
add a comment |
You need to show that the given relation is
reflexive
For all $(a, b) in X$, $ab = ba$. This is clearly true. Hence R is reflexive.symmetric
For all $(a, b), (c, d) in X$, suppose $(a, b) R(c, d).$ Then $ad = bc $ if and only if $cb = da$ if and only if $(c, d) R (a,b)$. Therefore, R is
symmetric.transitive
Now, see what you can do with the following: Take arbitrary $(a, b), (c, d), (e, f)$ and assume $(a, b)R (c, d)$, and $(c, d) R (e, f)$. So
$$ad = bc,quad text{and} quad cf = de$$ Now, we use a little algebra to show that this implies $af = be$, and hence $(a, b) R (e, f)$.
$ad = bc iff adf = bcf = b(cf).$ Also, we have $cf = de$. So $adf = b(de) iff af = be$, as desired!
The relation has all three properties, and hence, is by definition, an equivalence relation.
Hint: $$(a, b) R (c, d) ;text{ if and only if };frac{a}{b} = frac{c}{d}; text{ if and only if } ;ad = bc, ;;(bneq 0, dneq 0)$$
nice, thanks a lot!
– imre
Dec 18 '13 at 17:31
You're welcome!
– amWhy
Dec 18 '13 at 17:34
Helped me too. p.s. Crazy reputation.
– Det
Sep 21 '17 at 12:01
add a comment |
You need to show that the given relation is
reflexive
For all $(a, b) in X$, $ab = ba$. This is clearly true. Hence R is reflexive.symmetric
For all $(a, b), (c, d) in X$, suppose $(a, b) R(c, d).$ Then $ad = bc $ if and only if $cb = da$ if and only if $(c, d) R (a,b)$. Therefore, R is
symmetric.transitive
Now, see what you can do with the following: Take arbitrary $(a, b), (c, d), (e, f)$ and assume $(a, b)R (c, d)$, and $(c, d) R (e, f)$. So
$$ad = bc,quad text{and} quad cf = de$$ Now, we use a little algebra to show that this implies $af = be$, and hence $(a, b) R (e, f)$.
$ad = bc iff adf = bcf = b(cf).$ Also, we have $cf = de$. So $adf = b(de) iff af = be$, as desired!
The relation has all three properties, and hence, is by definition, an equivalence relation.
Hint: $$(a, b) R (c, d) ;text{ if and only if };frac{a}{b} = frac{c}{d}; text{ if and only if } ;ad = bc, ;;(bneq 0, dneq 0)$$
You need to show that the given relation is
reflexive
For all $(a, b) in X$, $ab = ba$. This is clearly true. Hence R is reflexive.symmetric
For all $(a, b), (c, d) in X$, suppose $(a, b) R(c, d).$ Then $ad = bc $ if and only if $cb = da$ if and only if $(c, d) R (a,b)$. Therefore, R is
symmetric.transitive
Now, see what you can do with the following: Take arbitrary $(a, b), (c, d), (e, f)$ and assume $(a, b)R (c, d)$, and $(c, d) R (e, f)$. So
$$ad = bc,quad text{and} quad cf = de$$ Now, we use a little algebra to show that this implies $af = be$, and hence $(a, b) R (e, f)$.
$ad = bc iff adf = bcf = b(cf).$ Also, we have $cf = de$. So $adf = b(de) iff af = be$, as desired!
The relation has all three properties, and hence, is by definition, an equivalence relation.
Hint: $$(a, b) R (c, d) ;text{ if and only if };frac{a}{b} = frac{c}{d}; text{ if and only if } ;ad = bc, ;;(bneq 0, dneq 0)$$
edited Dec 18 '13 at 17:29
answered Dec 18 '13 at 17:09
amWhyamWhy
192k28225439
192k28225439
nice, thanks a lot!
– imre
Dec 18 '13 at 17:31
You're welcome!
– amWhy
Dec 18 '13 at 17:34
Helped me too. p.s. Crazy reputation.
– Det
Sep 21 '17 at 12:01
add a comment |
nice, thanks a lot!
– imre
Dec 18 '13 at 17:31
You're welcome!
– amWhy
Dec 18 '13 at 17:34
Helped me too. p.s. Crazy reputation.
– Det
Sep 21 '17 at 12:01
nice, thanks a lot!
– imre
Dec 18 '13 at 17:31
nice, thanks a lot!
– imre
Dec 18 '13 at 17:31
You're welcome!
– amWhy
Dec 18 '13 at 17:34
You're welcome!
– amWhy
Dec 18 '13 at 17:34
Helped me too. p.s. Crazy reputation.
– Det
Sep 21 '17 at 12:01
Helped me too. p.s. Crazy reputation.
– Det
Sep 21 '17 at 12:01
add a comment |
To show this is an equivalence relation, you need to show, for pairs $(a,b),(c,d), (e,f)$:
i)$(a,b)R(a,b)$, for any pair $(a,b)$
ii) If $(a,b)R(c,d)$ , then $(c,d)R(a,b)$
iii)If $(a,b)R(c,d)$ and $(c,d)R(e,f)$ , then $(a,b)R(e,f)$.
Can you see how to it?
Let's set up i): We want to show that, for any pair $(a,b)$ , i.e., for any choice of integers $a,b$ , we have $(a,b)R(a,b)$. This means that, according to the definition of $R$, we have:
$(a,b)R(a,b)$ iff $ab=ba$ . Can you do the rest?
What numbers should I pick?
– imre
Dec 18 '13 at 17:13
It is supposed to be true for any choice of numbers. So try assuming that $a,b,c,d,e,f$ are any integers. Can you see how?
– user99680
Dec 18 '13 at 17:15
add a comment |
To show this is an equivalence relation, you need to show, for pairs $(a,b),(c,d), (e,f)$:
i)$(a,b)R(a,b)$, for any pair $(a,b)$
ii) If $(a,b)R(c,d)$ , then $(c,d)R(a,b)$
iii)If $(a,b)R(c,d)$ and $(c,d)R(e,f)$ , then $(a,b)R(e,f)$.
Can you see how to it?
Let's set up i): We want to show that, for any pair $(a,b)$ , i.e., for any choice of integers $a,b$ , we have $(a,b)R(a,b)$. This means that, according to the definition of $R$, we have:
$(a,b)R(a,b)$ iff $ab=ba$ . Can you do the rest?
What numbers should I pick?
– imre
Dec 18 '13 at 17:13
It is supposed to be true for any choice of numbers. So try assuming that $a,b,c,d,e,f$ are any integers. Can you see how?
– user99680
Dec 18 '13 at 17:15
add a comment |
To show this is an equivalence relation, you need to show, for pairs $(a,b),(c,d), (e,f)$:
i)$(a,b)R(a,b)$, for any pair $(a,b)$
ii) If $(a,b)R(c,d)$ , then $(c,d)R(a,b)$
iii)If $(a,b)R(c,d)$ and $(c,d)R(e,f)$ , then $(a,b)R(e,f)$.
Can you see how to it?
Let's set up i): We want to show that, for any pair $(a,b)$ , i.e., for any choice of integers $a,b$ , we have $(a,b)R(a,b)$. This means that, according to the definition of $R$, we have:
$(a,b)R(a,b)$ iff $ab=ba$ . Can you do the rest?
To show this is an equivalence relation, you need to show, for pairs $(a,b),(c,d), (e,f)$:
i)$(a,b)R(a,b)$, for any pair $(a,b)$
ii) If $(a,b)R(c,d)$ , then $(c,d)R(a,b)$
iii)If $(a,b)R(c,d)$ and $(c,d)R(e,f)$ , then $(a,b)R(e,f)$.
Can you see how to it?
Let's set up i): We want to show that, for any pair $(a,b)$ , i.e., for any choice of integers $a,b$ , we have $(a,b)R(a,b)$. This means that, according to the definition of $R$, we have:
$(a,b)R(a,b)$ iff $ab=ba$ . Can you do the rest?
edited Dec 18 '13 at 17:18
answered Dec 18 '13 at 17:12
user99680user99680
5,954821
5,954821
What numbers should I pick?
– imre
Dec 18 '13 at 17:13
It is supposed to be true for any choice of numbers. So try assuming that $a,b,c,d,e,f$ are any integers. Can you see how?
– user99680
Dec 18 '13 at 17:15
add a comment |
What numbers should I pick?
– imre
Dec 18 '13 at 17:13
It is supposed to be true for any choice of numbers. So try assuming that $a,b,c,d,e,f$ are any integers. Can you see how?
– user99680
Dec 18 '13 at 17:15
What numbers should I pick?
– imre
Dec 18 '13 at 17:13
What numbers should I pick?
– imre
Dec 18 '13 at 17:13
It is supposed to be true for any choice of numbers. So try assuming that $a,b,c,d,e,f$ are any integers. Can you see how?
– user99680
Dec 18 '13 at 17:15
It is supposed to be true for any choice of numbers. So try assuming that $a,b,c,d,e,f$ are any integers. Can you see how?
– user99680
Dec 18 '13 at 17:15
add a comment |
Consider that $$(a,b)R(c,d)quad text{ iff }quad ad = bcquad text{ iff }quadfrac{a}{b}=frac{c}{d}$$
and since $=$ is an equivalence relation, $R$ is an equivalence one, and $mathbb{Q}$ is an example of such classes.
add a comment |
Consider that $$(a,b)R(c,d)quad text{ iff }quad ad = bcquad text{ iff }quadfrac{a}{b}=frac{c}{d}$$
and since $=$ is an equivalence relation, $R$ is an equivalence one, and $mathbb{Q}$ is an example of such classes.
add a comment |
Consider that $$(a,b)R(c,d)quad text{ iff }quad ad = bcquad text{ iff }quadfrac{a}{b}=frac{c}{d}$$
and since $=$ is an equivalence relation, $R$ is an equivalence one, and $mathbb{Q}$ is an example of such classes.
Consider that $$(a,b)R(c,d)quad text{ iff }quad ad = bcquad text{ iff }quadfrac{a}{b}=frac{c}{d}$$
and since $=$ is an equivalence relation, $R$ is an equivalence one, and $mathbb{Q}$ is an example of such classes.
answered Dec 18 '13 at 17:21
Farshad NahangiFarshad Nahangi
45827
45827
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Some of your past answers have not been well-received, and you're in danger of being blocked from answering.
Please pay close attention to the following guidance:
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f611902%2fproof-that-a-b-mathrelr-c-d-iff-ad-bc-is-an-equivalence-relation%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
3
Have you tried showing the relation is reflexive, symmetric and transitive?
– user99680
Dec 18 '13 at 17:06
Note that $(a,b)R (c,d)$ iff $$frac{a}{b} = frac{c}{d}$$ Can you now guess what the set of equivalence classes is?
– Prahlad Vaidyanathan
Dec 18 '13 at 17:07
1
Hint:Recall the definition of rational numbers. $mathbb{Q}={frac{a}{b}|a,bin mathbb{Z},bne 0}$.
– Farshad Nahangi
Dec 18 '13 at 17:07