Can the empty set be the image of a function on $mathbb{N}$? [duplicate]
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This question already has an answer here:
Why is there no function with a nonempty domain and an empty range?
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I cannot find any example of function $f:mathbb{N}rightarrowmathbb{N}$ of which we can say that
$$
f(mathbb{N})=emptyset
$$
Does there exists any?
functions elementary-set-theory
marked as duplicate by Asaf Karagila♦
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Nov 26 at 15:22
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
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up vote
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This question already has an answer here:
Why is there no function with a nonempty domain and an empty range?
5 answers
I cannot find any example of function $f:mathbb{N}rightarrowmathbb{N}$ of which we can say that
$$
f(mathbb{N})=emptyset
$$
Does there exists any?
functions elementary-set-theory
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Nov 26 at 15:22
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
If $D_f=emptyset$
– hamam_Abdallah
Nov 25 at 18:33
2
By definition, the set $f(mathbb{N})$ contains each $f(n)$ for $nin mathbb{N}$.
– anomaly
Nov 26 at 0:46
You say "an empty set" but note that there is only one empty set: "the empty set."
– David Richerby
Nov 26 at 13:18
Not sure that this counts as a duplicate. The other question was a technical question of why the axiom of specification fails to claim "$Fsubset Atimes emptyset$ so that $F$ is a function" exists and required a far more technical and obtuse than this rather practical question. That's my two cents. But I'm certainly not going to vote to reopen.
– fleablood
Nov 26 at 16:45
add a comment |
up vote
5
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favorite
up vote
5
down vote
favorite
This question already has an answer here:
Why is there no function with a nonempty domain and an empty range?
5 answers
I cannot find any example of function $f:mathbb{N}rightarrowmathbb{N}$ of which we can say that
$$
f(mathbb{N})=emptyset
$$
Does there exists any?
functions elementary-set-theory
This question already has an answer here:
Why is there no function with a nonempty domain and an empty range?
5 answers
I cannot find any example of function $f:mathbb{N}rightarrowmathbb{N}$ of which we can say that
$$
f(mathbb{N})=emptyset
$$
Does there exists any?
This question already has an answer here:
Why is there no function with a nonempty domain and an empty range?
5 answers
functions elementary-set-theory
functions elementary-set-theory
edited Nov 26 at 15:02
user21820
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asked Nov 25 at 18:32
avan1235
1004
1004
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Nov 26 at 15:22
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Nov 26 at 15:22
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
If $D_f=emptyset$
– hamam_Abdallah
Nov 25 at 18:33
2
By definition, the set $f(mathbb{N})$ contains each $f(n)$ for $nin mathbb{N}$.
– anomaly
Nov 26 at 0:46
You say "an empty set" but note that there is only one empty set: "the empty set."
– David Richerby
Nov 26 at 13:18
Not sure that this counts as a duplicate. The other question was a technical question of why the axiom of specification fails to claim "$Fsubset Atimes emptyset$ so that $F$ is a function" exists and required a far more technical and obtuse than this rather practical question. That's my two cents. But I'm certainly not going to vote to reopen.
– fleablood
Nov 26 at 16:45
add a comment |
If $D_f=emptyset$
– hamam_Abdallah
Nov 25 at 18:33
2
By definition, the set $f(mathbb{N})$ contains each $f(n)$ for $nin mathbb{N}$.
– anomaly
Nov 26 at 0:46
You say "an empty set" but note that there is only one empty set: "the empty set."
– David Richerby
Nov 26 at 13:18
Not sure that this counts as a duplicate. The other question was a technical question of why the axiom of specification fails to claim "$Fsubset Atimes emptyset$ so that $F$ is a function" exists and required a far more technical and obtuse than this rather practical question. That's my two cents. But I'm certainly not going to vote to reopen.
– fleablood
Nov 26 at 16:45
If $D_f=emptyset$
– hamam_Abdallah
Nov 25 at 18:33
If $D_f=emptyset$
– hamam_Abdallah
Nov 25 at 18:33
2
2
By definition, the set $f(mathbb{N})$ contains each $f(n)$ for $nin mathbb{N}$.
– anomaly
Nov 26 at 0:46
By definition, the set $f(mathbb{N})$ contains each $f(n)$ for $nin mathbb{N}$.
– anomaly
Nov 26 at 0:46
You say "an empty set" but note that there is only one empty set: "the empty set."
– David Richerby
Nov 26 at 13:18
You say "an empty set" but note that there is only one empty set: "the empty set."
– David Richerby
Nov 26 at 13:18
Not sure that this counts as a duplicate. The other question was a technical question of why the axiom of specification fails to claim "$Fsubset Atimes emptyset$ so that $F$ is a function" exists and required a far more technical and obtuse than this rather practical question. That's my two cents. But I'm certainly not going to vote to reopen.
– fleablood
Nov 26 at 16:45
Not sure that this counts as a duplicate. The other question was a technical question of why the axiom of specification fails to claim "$Fsubset Atimes emptyset$ so that $F$ is a function" exists and required a far more technical and obtuse than this rather practical question. That's my two cents. But I'm certainly not going to vote to reopen.
– fleablood
Nov 26 at 16:45
add a comment |
5 Answers
5
active
oldest
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up vote
15
down vote
No. By definition of $f:A to B$, then for every $ain A$ then $f(a)$ must exist and $f(a) in B$. So if $A$ is not empty then $f(A)$ is not empty (although it can have a few as only one element.)
However it is possible that $A$ is empty in which case $f(A)$ is (obviously) also empty.
$f: emptyset to B$ is the empty function in this case.
=====
Or to put it really simple $f(1)$ has to be in the image so the image can't be empty.
You should say "No if the domain is non-empty.". As you've clearly proven in the last two lines the answer is "Yes".
– Ister
Nov 26 at 9:19
1
The OP states that the domain is $mathbb{N}$...
– Vincent
Nov 26 at 9:53
@Ister "You should say 'No if the domain is non-empty.'" Do you really think it needs to be proven that $Bbb N$ is non-empty?
– fleablood
Nov 26 at 16:21
@Vincent was your comment addressed at me or Ister. If at me, well, ... so? I've shown it is false for any non-empty domain. $Bbb N$ is a non-empty domain. So the answer to the OP's question is "no".
– fleablood
Nov 26 at 16:23
@fleablood the comment was directed at Ister.
– Vincent
Nov 26 at 16:25
|
show 1 more comment
up vote
9
down vote
There is no such function: in particular, the only function whose image is the empty set is the empty function, whose domain is also the empty set. In particular, $f(mathbb{N})$ cannot be the empty set, because contains $f(1)$.
add a comment |
up vote
7
down vote
Well, the definition state that for any x natural number there exists a uniquely determined natural number y such as f(x)=y. Since f(N) has no elements, it means that there doesn’t exist any y such as f(x)=y. Contradiction! So such functions do not exist :) The image of a function has at least one element as Df has at least 1 element.
add a comment |
up vote
3
down vote
Since for every $ain mathbb{N}$ the $f(a)$ is also in $mathbb{N}$ we see that $f(mathbb{N})ne emptyset $ so there is no such function.
add a comment |
up vote
1
down vote
This case is not possible since, by definition of range of a function, $f(a)$ should belong to the codomain for each $a$ in the domain.
add a comment |
5 Answers
5
active
oldest
votes
5 Answers
5
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
15
down vote
No. By definition of $f:A to B$, then for every $ain A$ then $f(a)$ must exist and $f(a) in B$. So if $A$ is not empty then $f(A)$ is not empty (although it can have a few as only one element.)
However it is possible that $A$ is empty in which case $f(A)$ is (obviously) also empty.
$f: emptyset to B$ is the empty function in this case.
=====
Or to put it really simple $f(1)$ has to be in the image so the image can't be empty.
You should say "No if the domain is non-empty.". As you've clearly proven in the last two lines the answer is "Yes".
– Ister
Nov 26 at 9:19
1
The OP states that the domain is $mathbb{N}$...
– Vincent
Nov 26 at 9:53
@Ister "You should say 'No if the domain is non-empty.'" Do you really think it needs to be proven that $Bbb N$ is non-empty?
– fleablood
Nov 26 at 16:21
@Vincent was your comment addressed at me or Ister. If at me, well, ... so? I've shown it is false for any non-empty domain. $Bbb N$ is a non-empty domain. So the answer to the OP's question is "no".
– fleablood
Nov 26 at 16:23
@fleablood the comment was directed at Ister.
– Vincent
Nov 26 at 16:25
|
show 1 more comment
up vote
15
down vote
No. By definition of $f:A to B$, then for every $ain A$ then $f(a)$ must exist and $f(a) in B$. So if $A$ is not empty then $f(A)$ is not empty (although it can have a few as only one element.)
However it is possible that $A$ is empty in which case $f(A)$ is (obviously) also empty.
$f: emptyset to B$ is the empty function in this case.
=====
Or to put it really simple $f(1)$ has to be in the image so the image can't be empty.
You should say "No if the domain is non-empty.". As you've clearly proven in the last two lines the answer is "Yes".
– Ister
Nov 26 at 9:19
1
The OP states that the domain is $mathbb{N}$...
– Vincent
Nov 26 at 9:53
@Ister "You should say 'No if the domain is non-empty.'" Do you really think it needs to be proven that $Bbb N$ is non-empty?
– fleablood
Nov 26 at 16:21
@Vincent was your comment addressed at me or Ister. If at me, well, ... so? I've shown it is false for any non-empty domain. $Bbb N$ is a non-empty domain. So the answer to the OP's question is "no".
– fleablood
Nov 26 at 16:23
@fleablood the comment was directed at Ister.
– Vincent
Nov 26 at 16:25
|
show 1 more comment
up vote
15
down vote
up vote
15
down vote
No. By definition of $f:A to B$, then for every $ain A$ then $f(a)$ must exist and $f(a) in B$. So if $A$ is not empty then $f(A)$ is not empty (although it can have a few as only one element.)
However it is possible that $A$ is empty in which case $f(A)$ is (obviously) also empty.
$f: emptyset to B$ is the empty function in this case.
=====
Or to put it really simple $f(1)$ has to be in the image so the image can't be empty.
No. By definition of $f:A to B$, then for every $ain A$ then $f(a)$ must exist and $f(a) in B$. So if $A$ is not empty then $f(A)$ is not empty (although it can have a few as only one element.)
However it is possible that $A$ is empty in which case $f(A)$ is (obviously) also empty.
$f: emptyset to B$ is the empty function in this case.
=====
Or to put it really simple $f(1)$ has to be in the image so the image can't be empty.
edited Nov 26 at 22:19
answered Nov 25 at 18:42
fleablood
66.4k22684
66.4k22684
You should say "No if the domain is non-empty.". As you've clearly proven in the last two lines the answer is "Yes".
– Ister
Nov 26 at 9:19
1
The OP states that the domain is $mathbb{N}$...
– Vincent
Nov 26 at 9:53
@Ister "You should say 'No if the domain is non-empty.'" Do you really think it needs to be proven that $Bbb N$ is non-empty?
– fleablood
Nov 26 at 16:21
@Vincent was your comment addressed at me or Ister. If at me, well, ... so? I've shown it is false for any non-empty domain. $Bbb N$ is a non-empty domain. So the answer to the OP's question is "no".
– fleablood
Nov 26 at 16:23
@fleablood the comment was directed at Ister.
– Vincent
Nov 26 at 16:25
|
show 1 more comment
You should say "No if the domain is non-empty.". As you've clearly proven in the last two lines the answer is "Yes".
– Ister
Nov 26 at 9:19
1
The OP states that the domain is $mathbb{N}$...
– Vincent
Nov 26 at 9:53
@Ister "You should say 'No if the domain is non-empty.'" Do you really think it needs to be proven that $Bbb N$ is non-empty?
– fleablood
Nov 26 at 16:21
@Vincent was your comment addressed at me or Ister. If at me, well, ... so? I've shown it is false for any non-empty domain. $Bbb N$ is a non-empty domain. So the answer to the OP's question is "no".
– fleablood
Nov 26 at 16:23
@fleablood the comment was directed at Ister.
– Vincent
Nov 26 at 16:25
You should say "No if the domain is non-empty.". As you've clearly proven in the last two lines the answer is "Yes".
– Ister
Nov 26 at 9:19
You should say "No if the domain is non-empty.". As you've clearly proven in the last two lines the answer is "Yes".
– Ister
Nov 26 at 9:19
1
1
The OP states that the domain is $mathbb{N}$...
– Vincent
Nov 26 at 9:53
The OP states that the domain is $mathbb{N}$...
– Vincent
Nov 26 at 9:53
@Ister "You should say 'No if the domain is non-empty.'" Do you really think it needs to be proven that $Bbb N$ is non-empty?
– fleablood
Nov 26 at 16:21
@Ister "You should say 'No if the domain is non-empty.'" Do you really think it needs to be proven that $Bbb N$ is non-empty?
– fleablood
Nov 26 at 16:21
@Vincent was your comment addressed at me or Ister. If at me, well, ... so? I've shown it is false for any non-empty domain. $Bbb N$ is a non-empty domain. So the answer to the OP's question is "no".
– fleablood
Nov 26 at 16:23
@Vincent was your comment addressed at me or Ister. If at me, well, ... so? I've shown it is false for any non-empty domain. $Bbb N$ is a non-empty domain. So the answer to the OP's question is "no".
– fleablood
Nov 26 at 16:23
@fleablood the comment was directed at Ister.
– Vincent
Nov 26 at 16:25
@fleablood the comment was directed at Ister.
– Vincent
Nov 26 at 16:25
|
show 1 more comment
up vote
9
down vote
There is no such function: in particular, the only function whose image is the empty set is the empty function, whose domain is also the empty set. In particular, $f(mathbb{N})$ cannot be the empty set, because contains $f(1)$.
add a comment |
up vote
9
down vote
There is no such function: in particular, the only function whose image is the empty set is the empty function, whose domain is also the empty set. In particular, $f(mathbb{N})$ cannot be the empty set, because contains $f(1)$.
add a comment |
up vote
9
down vote
up vote
9
down vote
There is no such function: in particular, the only function whose image is the empty set is the empty function, whose domain is also the empty set. In particular, $f(mathbb{N})$ cannot be the empty set, because contains $f(1)$.
There is no such function: in particular, the only function whose image is the empty set is the empty function, whose domain is also the empty set. In particular, $f(mathbb{N})$ cannot be the empty set, because contains $f(1)$.
answered Nov 25 at 18:36
user3482749
1,779411
1,779411
add a comment |
add a comment |
up vote
7
down vote
Well, the definition state that for any x natural number there exists a uniquely determined natural number y such as f(x)=y. Since f(N) has no elements, it means that there doesn’t exist any y such as f(x)=y. Contradiction! So such functions do not exist :) The image of a function has at least one element as Df has at least 1 element.
add a comment |
up vote
7
down vote
Well, the definition state that for any x natural number there exists a uniquely determined natural number y such as f(x)=y. Since f(N) has no elements, it means that there doesn’t exist any y such as f(x)=y. Contradiction! So such functions do not exist :) The image of a function has at least one element as Df has at least 1 element.
add a comment |
up vote
7
down vote
up vote
7
down vote
Well, the definition state that for any x natural number there exists a uniquely determined natural number y such as f(x)=y. Since f(N) has no elements, it means that there doesn’t exist any y such as f(x)=y. Contradiction! So such functions do not exist :) The image of a function has at least one element as Df has at least 1 element.
Well, the definition state that for any x natural number there exists a uniquely determined natural number y such as f(x)=y. Since f(N) has no elements, it means that there doesn’t exist any y such as f(x)=y. Contradiction! So such functions do not exist :) The image of a function has at least one element as Df has at least 1 element.
answered Nov 25 at 18:39
furfur
854
854
add a comment |
add a comment |
up vote
3
down vote
Since for every $ain mathbb{N}$ the $f(a)$ is also in $mathbb{N}$ we see that $f(mathbb{N})ne emptyset $ so there is no such function.
add a comment |
up vote
3
down vote
Since for every $ain mathbb{N}$ the $f(a)$ is also in $mathbb{N}$ we see that $f(mathbb{N})ne emptyset $ so there is no such function.
add a comment |
up vote
3
down vote
up vote
3
down vote
Since for every $ain mathbb{N}$ the $f(a)$ is also in $mathbb{N}$ we see that $f(mathbb{N})ne emptyset $ so there is no such function.
Since for every $ain mathbb{N}$ the $f(a)$ is also in $mathbb{N}$ we see that $f(mathbb{N})ne emptyset $ so there is no such function.
answered Nov 25 at 18:34
greedoid
35k114489
35k114489
add a comment |
add a comment |
up vote
1
down vote
This case is not possible since, by definition of range of a function, $f(a)$ should belong to the codomain for each $a$ in the domain.
add a comment |
up vote
1
down vote
This case is not possible since, by definition of range of a function, $f(a)$ should belong to the codomain for each $a$ in the domain.
add a comment |
up vote
1
down vote
up vote
1
down vote
This case is not possible since, by definition of range of a function, $f(a)$ should belong to the codomain for each $a$ in the domain.
This case is not possible since, by definition of range of a function, $f(a)$ should belong to the codomain for each $a$ in the domain.
edited 2 hours ago
Brahadeesh
5,78341957
5,78341957
answered Nov 25 at 18:56
MANI SHANKAR PANDEY
112
112
add a comment |
add a comment |
If $D_f=emptyset$
– hamam_Abdallah
Nov 25 at 18:33
2
By definition, the set $f(mathbb{N})$ contains each $f(n)$ for $nin mathbb{N}$.
– anomaly
Nov 26 at 0:46
You say "an empty set" but note that there is only one empty set: "the empty set."
– David Richerby
Nov 26 at 13:18
Not sure that this counts as a duplicate. The other question was a technical question of why the axiom of specification fails to claim "$Fsubset Atimes emptyset$ so that $F$ is a function" exists and required a far more technical and obtuse than this rather practical question. That's my two cents. But I'm certainly not going to vote to reopen.
– fleablood
Nov 26 at 16:45