Union of a finite set and a countably infinite set is countably infinite
up vote
2
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Ok, here is the problem statement:
Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.
This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!
elementary-set-theory
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
asked Apr 7 '13 at 23:26
user69839
6327
add a comment |
up vote
2
down vote
favorite
Ok, here is the problem statement:
Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.
This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!
elementary-set-theory
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
asked Apr 7 '13 at 23:26
user69839
6327
What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30
add a comment |
up vote
2
down vote
favorite
up vote
2
down vote
favorite
Ok, here is the problem statement:
Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.
This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!
elementary-set-theory
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
asked Apr 7 '13 at 23:26
user69839
6327
Ok, here is the problem statement:
Prove that if $S$ is any finite set of real numbers, then the union of $S$ and the integers is countably infinite.
This seems pretty obvious to me, knowing that 2 countable sets are countable. But is there some step by step way to prove this? Like do I need to prove bijectivity or something? Thanks!
elementary-set-theory
elementary-set-theory
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
asked Apr 7 '13 at 23:26
user69839
6327
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
asked Apr 7 '13 at 23:26
user69839
6327
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
edited Apr 7 '13 at 23:30
Zev Chonoles
109k16225422
109k16225422
asked Apr 7 '13 at 23:26
user69839
6327
asked Apr 7 '13 at 23:26
user69839
6327
asked Apr 7 '13 at 23:26
user69839
6327
6327
What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30
add a comment |
What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30
What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30
What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30
add a comment |
2 Answers
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Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.
Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
$$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...
answered Apr 7 '13 at 23:31
Berci
59.4k23672
Thank you! I think I got it
– user69839
Apr 7 '13 at 23:34
add a comment |
up vote
1
down vote
It is simple to prove that the countable union of countable sets is countable:
Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
4
down vote
accepted
Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.
Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
$$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...
answered Apr 7 '13 at 23:31
Berci
59.4k23672
Thank you! I think I got it
– user69839
Apr 7 '13 at 23:34
add a comment |
up vote
4
down vote
accepted
Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.
Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
$$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...
answered Apr 7 '13 at 23:31
Berci
59.4k23672
Thank you! I think I got it
– user69839
Apr 7 '13 at 23:34
add a comment |
up vote
4
down vote
accepted
up vote
4
down vote
accepted
Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.
Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
$$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...
answered Apr 7 '13 at 23:31
Berci
59.4k23672
Yes, give a bijection between $Bbb N$ and $Bbb Zcup S$, that is a sequence in the latter which uses each element exactly once.
Say, $SsetminusBbb Z={s_1,..,s_k}$. Then for example this sequence gives a bijection:
$$(s_1,s_2,...,s_k,0,1,-1,2,-2,3,-3,4,-4,dots)$$
That is, $1$ will be mapped to $s_1$, $k+1$ will be mapped to $0$, $k+2$ to $1$, and so on...
answered Apr 7 '13 at 23:31
Berci
59.4k23672
answered Apr 7 '13 at 23:31
Berci
59.4k23672
answered Apr 7 '13 at 23:31
Berci
59.4k23672
answered Apr 7 '13 at 23:31
Berci
59.4k23672
59.4k23672
Thank you! I think I got it
– user69839
Apr 7 '13 at 23:34
add a comment |
Thank you! I think I got it
– user69839
Apr 7 '13 at 23:34
Thank you! I think I got it
– user69839
Apr 7 '13 at 23:34
Thank you! I think I got it
– user69839
Apr 7 '13 at 23:34
add a comment |
up vote
1
down vote
It is simple to prove that the countable union of countable sets is countable:
Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
add a comment |
up vote
1
down vote
It is simple to prove that the countable union of countable sets is countable:
Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
add a comment |
up vote
1
down vote
up vote
1
down vote
It is simple to prove that the countable union of countable sets is countable:
Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
It is simple to prove that the countable union of countable sets is countable:
Lay out each set in a line (if finite, just repeat over and over), and then go over the elements $e_{i j}$ diagonally: First 0, 0; then 0, 1 and 1, 0; then 2, 0 and 1, 1 and 0, 2; ... If an element has shown up already, skip it. This gives a biyection between $mathbb{N}$ and the union, unless the union is finite.
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
answered Apr 7 '13 at 23:44
vonbrand
19.8k63058
19.8k63058
add a comment |
add a comment |
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What are the options for the cardinality for our set $A=Scupmathbb Z$? It's certainly not finite, since $|A|geq |mathbb Z|$. Could it be uncountably infinite?
– Ian Coley
Apr 7 '13 at 23:30