Does invertability and closure imply identity?
Sorry if this is a basic question or I'm overthinking it, but if an algebraic structure has inverse elements (or at least for a member $a$), that means $a^{-1}a=e$, and if there's closure then e is an element of the set. So in the case of defining a group, for instance, why do we need to include identity? Is it already implied?
abstract-algebra binary-operations
add a comment |
Sorry if this is a basic question or I'm overthinking it, but if an algebraic structure has inverse elements (or at least for a member $a$), that means $a^{-1}a=e$, and if there's closure then e is an element of the set. So in the case of defining a group, for instance, why do we need to include identity? Is it already implied?
abstract-algebra binary-operations
What alternate axiom(s) are you proposing, precisely?
– lulu
3 hours ago
@lulu none, I was wondering why we can't remove this axiom because it appears to already be implied. I think I get it now though.
– Benjamin Thoburn
3 hours ago
Because you need that $, ae=ea=a,$ for all $,a.$
– Somos
2 hours ago
add a comment |
Sorry if this is a basic question or I'm overthinking it, but if an algebraic structure has inverse elements (or at least for a member $a$), that means $a^{-1}a=e$, and if there's closure then e is an element of the set. So in the case of defining a group, for instance, why do we need to include identity? Is it already implied?
abstract-algebra binary-operations
Sorry if this is a basic question or I'm overthinking it, but if an algebraic structure has inverse elements (or at least for a member $a$), that means $a^{-1}a=e$, and if there's closure then e is an element of the set. So in the case of defining a group, for instance, why do we need to include identity? Is it already implied?
abstract-algebra binary-operations
abstract-algebra binary-operations
edited 3 hours ago
José Carlos Santos
150k22120221
150k22120221
asked 3 hours ago
Benjamin Thoburn
15210
15210
What alternate axiom(s) are you proposing, precisely?
– lulu
3 hours ago
@lulu none, I was wondering why we can't remove this axiom because it appears to already be implied. I think I get it now though.
– Benjamin Thoburn
3 hours ago
Because you need that $, ae=ea=a,$ for all $,a.$
– Somos
2 hours ago
add a comment |
What alternate axiom(s) are you proposing, precisely?
– lulu
3 hours ago
@lulu none, I was wondering why we can't remove this axiom because it appears to already be implied. I think I get it now though.
– Benjamin Thoburn
3 hours ago
Because you need that $, ae=ea=a,$ for all $,a.$
– Somos
2 hours ago
What alternate axiom(s) are you proposing, precisely?
– lulu
3 hours ago
What alternate axiom(s) are you proposing, precisely?
– lulu
3 hours ago
@lulu none, I was wondering why we can't remove this axiom because it appears to already be implied. I think I get it now though.
– Benjamin Thoburn
3 hours ago
@lulu none, I was wondering why we can't remove this axiom because it appears to already be implied. I think I get it now though.
– Benjamin Thoburn
3 hours ago
Because you need that $, ae=ea=a,$ for all $,a.$
– Somos
2 hours ago
Because you need that $, ae=ea=a,$ for all $,a.$
– Somos
2 hours ago
add a comment |
2 Answers
2
active
oldest
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Because if we were not essuming the existence of the identity element $e$, we would not be able to express the idea that $a^{-1}a=e$.
1
+1 One can express the idea that $xa=b$ has a solution for all $a$ and $b$, though. Together with identities (and associativity) this is equivalent to having left inverses, but it makes sense to speak of even in structures without identities,
– Henning Makholm
3 hours ago
1
(And if both $xa=b$ and $ax=b$ always have solutions, then you automatically have identities and inverses).
– Henning Makholm
3 hours ago
To check my understanding, while invertability does require identity it adds something extra, so it's worth specifying. But indentity is a basis for invertability though so it too has to be specified... I suppose I'm still a bit confused though since while you do need identity for invertability I don't see why you have to say it explicitly as an axiom because it's not adding anything new.
– Benjamin Thoburn
2 hours ago
If you assert that it adds nothing new, please explan how to express the existence of an inverse of each element without it. Unless that the existence of an inverse becomes$$(forall ain G)(exists a'in G)(forall bin G):a'ab=ba'a=b,$$which is a bit heavy in my opinion.
– José Carlos Santos
2 hours ago
What I'm trying to get at isn't identity existing for invertability, but rather that it seems that inverse is a sort of "compound" statement, asserting both that inverse elements exist but also with this statement you can conclude there has to be an identity, because without it, as you said, you couldn't even define invertability. In your answer you said WHY identity has to be there but what I'm asking about is if it has to be there in one way WHY add it in a second way? It feels unnessesary to be redundant, introducing this as its own new and seperate axiom.I hope I'm explaining myself well?
– Benjamin Thoburn
34 mins ago
|
show 2 more comments
It's a reasonable question whether identity needs to be its own axiom. The inverse axiom requires that $a^{-1}$ exist for each $a$ such that $a^{-1} a=a a^{-1} = e$, which of course uses the identity in the definition. Instead, you might extend the inverse axiom as follows:
For each $ain G$, there exists an element $a^{-1}in G$, such that $a^{-1}a=aa^{-1}$ (call the result $e_a$), and such that for all $bin G$, $e_a b=be_a =b$.
This seems to be weaker, allowing multiple distinct identity-ish elements, but it's not hard to show that they must actually all be the same element. So extending the inverse axiom in this way gets you back to the same definition of a group, except for one thing: without the identity axiom, the empty set would vacuously constitute a group.
1
$e_a e_b=e_b=e_a$
– Zachary Selk
3 hours ago
add a comment |
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2 Answers
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2 Answers
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Because if we were not essuming the existence of the identity element $e$, we would not be able to express the idea that $a^{-1}a=e$.
1
+1 One can express the idea that $xa=b$ has a solution for all $a$ and $b$, though. Together with identities (and associativity) this is equivalent to having left inverses, but it makes sense to speak of even in structures without identities,
– Henning Makholm
3 hours ago
1
(And if both $xa=b$ and $ax=b$ always have solutions, then you automatically have identities and inverses).
– Henning Makholm
3 hours ago
To check my understanding, while invertability does require identity it adds something extra, so it's worth specifying. But indentity is a basis for invertability though so it too has to be specified... I suppose I'm still a bit confused though since while you do need identity for invertability I don't see why you have to say it explicitly as an axiom because it's not adding anything new.
– Benjamin Thoburn
2 hours ago
If you assert that it adds nothing new, please explan how to express the existence of an inverse of each element without it. Unless that the existence of an inverse becomes$$(forall ain G)(exists a'in G)(forall bin G):a'ab=ba'a=b,$$which is a bit heavy in my opinion.
– José Carlos Santos
2 hours ago
What I'm trying to get at isn't identity existing for invertability, but rather that it seems that inverse is a sort of "compound" statement, asserting both that inverse elements exist but also with this statement you can conclude there has to be an identity, because without it, as you said, you couldn't even define invertability. In your answer you said WHY identity has to be there but what I'm asking about is if it has to be there in one way WHY add it in a second way? It feels unnessesary to be redundant, introducing this as its own new and seperate axiom.I hope I'm explaining myself well?
– Benjamin Thoburn
34 mins ago
|
show 2 more comments
Because if we were not essuming the existence of the identity element $e$, we would not be able to express the idea that $a^{-1}a=e$.
1
+1 One can express the idea that $xa=b$ has a solution for all $a$ and $b$, though. Together with identities (and associativity) this is equivalent to having left inverses, but it makes sense to speak of even in structures without identities,
– Henning Makholm
3 hours ago
1
(And if both $xa=b$ and $ax=b$ always have solutions, then you automatically have identities and inverses).
– Henning Makholm
3 hours ago
To check my understanding, while invertability does require identity it adds something extra, so it's worth specifying. But indentity is a basis for invertability though so it too has to be specified... I suppose I'm still a bit confused though since while you do need identity for invertability I don't see why you have to say it explicitly as an axiom because it's not adding anything new.
– Benjamin Thoburn
2 hours ago
If you assert that it adds nothing new, please explan how to express the existence of an inverse of each element without it. Unless that the existence of an inverse becomes$$(forall ain G)(exists a'in G)(forall bin G):a'ab=ba'a=b,$$which is a bit heavy in my opinion.
– José Carlos Santos
2 hours ago
What I'm trying to get at isn't identity existing for invertability, but rather that it seems that inverse is a sort of "compound" statement, asserting both that inverse elements exist but also with this statement you can conclude there has to be an identity, because without it, as you said, you couldn't even define invertability. In your answer you said WHY identity has to be there but what I'm asking about is if it has to be there in one way WHY add it in a second way? It feels unnessesary to be redundant, introducing this as its own new and seperate axiom.I hope I'm explaining myself well?
– Benjamin Thoburn
34 mins ago
|
show 2 more comments
Because if we were not essuming the existence of the identity element $e$, we would not be able to express the idea that $a^{-1}a=e$.
Because if we were not essuming the existence of the identity element $e$, we would not be able to express the idea that $a^{-1}a=e$.
answered 3 hours ago
José Carlos Santos
150k22120221
150k22120221
1
+1 One can express the idea that $xa=b$ has a solution for all $a$ and $b$, though. Together with identities (and associativity) this is equivalent to having left inverses, but it makes sense to speak of even in structures without identities,
– Henning Makholm
3 hours ago
1
(And if both $xa=b$ and $ax=b$ always have solutions, then you automatically have identities and inverses).
– Henning Makholm
3 hours ago
To check my understanding, while invertability does require identity it adds something extra, so it's worth specifying. But indentity is a basis for invertability though so it too has to be specified... I suppose I'm still a bit confused though since while you do need identity for invertability I don't see why you have to say it explicitly as an axiom because it's not adding anything new.
– Benjamin Thoburn
2 hours ago
If you assert that it adds nothing new, please explan how to express the existence of an inverse of each element without it. Unless that the existence of an inverse becomes$$(forall ain G)(exists a'in G)(forall bin G):a'ab=ba'a=b,$$which is a bit heavy in my opinion.
– José Carlos Santos
2 hours ago
What I'm trying to get at isn't identity existing for invertability, but rather that it seems that inverse is a sort of "compound" statement, asserting both that inverse elements exist but also with this statement you can conclude there has to be an identity, because without it, as you said, you couldn't even define invertability. In your answer you said WHY identity has to be there but what I'm asking about is if it has to be there in one way WHY add it in a second way? It feels unnessesary to be redundant, introducing this as its own new and seperate axiom.I hope I'm explaining myself well?
– Benjamin Thoburn
34 mins ago
|
show 2 more comments
1
+1 One can express the idea that $xa=b$ has a solution for all $a$ and $b$, though. Together with identities (and associativity) this is equivalent to having left inverses, but it makes sense to speak of even in structures without identities,
– Henning Makholm
3 hours ago
1
(And if both $xa=b$ and $ax=b$ always have solutions, then you automatically have identities and inverses).
– Henning Makholm
3 hours ago
To check my understanding, while invertability does require identity it adds something extra, so it's worth specifying. But indentity is a basis for invertability though so it too has to be specified... I suppose I'm still a bit confused though since while you do need identity for invertability I don't see why you have to say it explicitly as an axiom because it's not adding anything new.
– Benjamin Thoburn
2 hours ago
If you assert that it adds nothing new, please explan how to express the existence of an inverse of each element without it. Unless that the existence of an inverse becomes$$(forall ain G)(exists a'in G)(forall bin G):a'ab=ba'a=b,$$which is a bit heavy in my opinion.
– José Carlos Santos
2 hours ago
What I'm trying to get at isn't identity existing for invertability, but rather that it seems that inverse is a sort of "compound" statement, asserting both that inverse elements exist but also with this statement you can conclude there has to be an identity, because without it, as you said, you couldn't even define invertability. In your answer you said WHY identity has to be there but what I'm asking about is if it has to be there in one way WHY add it in a second way? It feels unnessesary to be redundant, introducing this as its own new and seperate axiom.I hope I'm explaining myself well?
– Benjamin Thoburn
34 mins ago
1
1
+1 One can express the idea that $xa=b$ has a solution for all $a$ and $b$, though. Together with identities (and associativity) this is equivalent to having left inverses, but it makes sense to speak of even in structures without identities,
– Henning Makholm
3 hours ago
+1 One can express the idea that $xa=b$ has a solution for all $a$ and $b$, though. Together with identities (and associativity) this is equivalent to having left inverses, but it makes sense to speak of even in structures without identities,
– Henning Makholm
3 hours ago
1
1
(And if both $xa=b$ and $ax=b$ always have solutions, then you automatically have identities and inverses).
– Henning Makholm
3 hours ago
(And if both $xa=b$ and $ax=b$ always have solutions, then you automatically have identities and inverses).
– Henning Makholm
3 hours ago
To check my understanding, while invertability does require identity it adds something extra, so it's worth specifying. But indentity is a basis for invertability though so it too has to be specified... I suppose I'm still a bit confused though since while you do need identity for invertability I don't see why you have to say it explicitly as an axiom because it's not adding anything new.
– Benjamin Thoburn
2 hours ago
To check my understanding, while invertability does require identity it adds something extra, so it's worth specifying. But indentity is a basis for invertability though so it too has to be specified... I suppose I'm still a bit confused though since while you do need identity for invertability I don't see why you have to say it explicitly as an axiom because it's not adding anything new.
– Benjamin Thoburn
2 hours ago
If you assert that it adds nothing new, please explan how to express the existence of an inverse of each element without it. Unless that the existence of an inverse becomes$$(forall ain G)(exists a'in G)(forall bin G):a'ab=ba'a=b,$$which is a bit heavy in my opinion.
– José Carlos Santos
2 hours ago
If you assert that it adds nothing new, please explan how to express the existence of an inverse of each element without it. Unless that the existence of an inverse becomes$$(forall ain G)(exists a'in G)(forall bin G):a'ab=ba'a=b,$$which is a bit heavy in my opinion.
– José Carlos Santos
2 hours ago
What I'm trying to get at isn't identity existing for invertability, but rather that it seems that inverse is a sort of "compound" statement, asserting both that inverse elements exist but also with this statement you can conclude there has to be an identity, because without it, as you said, you couldn't even define invertability. In your answer you said WHY identity has to be there but what I'm asking about is if it has to be there in one way WHY add it in a second way? It feels unnessesary to be redundant, introducing this as its own new and seperate axiom.I hope I'm explaining myself well?
– Benjamin Thoburn
34 mins ago
What I'm trying to get at isn't identity existing for invertability, but rather that it seems that inverse is a sort of "compound" statement, asserting both that inverse elements exist but also with this statement you can conclude there has to be an identity, because without it, as you said, you couldn't even define invertability. In your answer you said WHY identity has to be there but what I'm asking about is if it has to be there in one way WHY add it in a second way? It feels unnessesary to be redundant, introducing this as its own new and seperate axiom.I hope I'm explaining myself well?
– Benjamin Thoburn
34 mins ago
|
show 2 more comments
It's a reasonable question whether identity needs to be its own axiom. The inverse axiom requires that $a^{-1}$ exist for each $a$ such that $a^{-1} a=a a^{-1} = e$, which of course uses the identity in the definition. Instead, you might extend the inverse axiom as follows:
For each $ain G$, there exists an element $a^{-1}in G$, such that $a^{-1}a=aa^{-1}$ (call the result $e_a$), and such that for all $bin G$, $e_a b=be_a =b$.
This seems to be weaker, allowing multiple distinct identity-ish elements, but it's not hard to show that they must actually all be the same element. So extending the inverse axiom in this way gets you back to the same definition of a group, except for one thing: without the identity axiom, the empty set would vacuously constitute a group.
1
$e_a e_b=e_b=e_a$
– Zachary Selk
3 hours ago
add a comment |
It's a reasonable question whether identity needs to be its own axiom. The inverse axiom requires that $a^{-1}$ exist for each $a$ such that $a^{-1} a=a a^{-1} = e$, which of course uses the identity in the definition. Instead, you might extend the inverse axiom as follows:
For each $ain G$, there exists an element $a^{-1}in G$, such that $a^{-1}a=aa^{-1}$ (call the result $e_a$), and such that for all $bin G$, $e_a b=be_a =b$.
This seems to be weaker, allowing multiple distinct identity-ish elements, but it's not hard to show that they must actually all be the same element. So extending the inverse axiom in this way gets you back to the same definition of a group, except for one thing: without the identity axiom, the empty set would vacuously constitute a group.
1
$e_a e_b=e_b=e_a$
– Zachary Selk
3 hours ago
add a comment |
It's a reasonable question whether identity needs to be its own axiom. The inverse axiom requires that $a^{-1}$ exist for each $a$ such that $a^{-1} a=a a^{-1} = e$, which of course uses the identity in the definition. Instead, you might extend the inverse axiom as follows:
For each $ain G$, there exists an element $a^{-1}in G$, such that $a^{-1}a=aa^{-1}$ (call the result $e_a$), and such that for all $bin G$, $e_a b=be_a =b$.
This seems to be weaker, allowing multiple distinct identity-ish elements, but it's not hard to show that they must actually all be the same element. So extending the inverse axiom in this way gets you back to the same definition of a group, except for one thing: without the identity axiom, the empty set would vacuously constitute a group.
It's a reasonable question whether identity needs to be its own axiom. The inverse axiom requires that $a^{-1}$ exist for each $a$ such that $a^{-1} a=a a^{-1} = e$, which of course uses the identity in the definition. Instead, you might extend the inverse axiom as follows:
For each $ain G$, there exists an element $a^{-1}in G$, such that $a^{-1}a=aa^{-1}$ (call the result $e_a$), and such that for all $bin G$, $e_a b=be_a =b$.
This seems to be weaker, allowing multiple distinct identity-ish elements, but it's not hard to show that they must actually all be the same element. So extending the inverse axiom in this way gets you back to the same definition of a group, except for one thing: without the identity axiom, the empty set would vacuously constitute a group.
answered 3 hours ago
mjqxxxx
31.1k23985
31.1k23985
1
$e_a e_b=e_b=e_a$
– Zachary Selk
3 hours ago
add a comment |
1
$e_a e_b=e_b=e_a$
– Zachary Selk
3 hours ago
1
1
$e_a e_b=e_b=e_a$
– Zachary Selk
3 hours ago
$e_a e_b=e_b=e_a$
– Zachary Selk
3 hours ago
add a comment |
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What alternate axiom(s) are you proposing, precisely?
– lulu
3 hours ago
@lulu none, I was wondering why we can't remove this axiom because it appears to already be implied. I think I get it now though.
– Benjamin Thoburn
3 hours ago
Because you need that $, ae=ea=a,$ for all $,a.$
– Somos
2 hours ago