Existence and uniqueness of adjoints with respect to pairings
$begingroup$
Let $V,W,L$ be $R$-modules over a commutative ring $R$. A pairing is an $R$-linear map $Votimes Wto L$. An adjoint of an endomorphism $f:Vto V$ w.r.t a pairing $Votimes Woverset{g}{to}L$ is an endomorphism $f^{dagger ^g}:Wto W$ such that $g(fotimes 1)=g(1otimes f^{dagger^g})$.
When do such things exist and when are they unique? That is, what assumptions are needed on the modules involved, the pairing $g$, and $f$ itself? What if we suppose $g$ is a perfect pairing? (Perhaps this questions is as easily answerable for nice monoidal categories.)
For instance the adjugate of a linear endomorphism can be defined as an adjoint with respect to a canonical pairing. I would simply like to understand which "dualizability" assumptions are needed and where.
linear-algebra commutative-algebra category-theory adjoint-operators monoidal-categories
$endgroup$
add a comment |
$begingroup$
Let $V,W,L$ be $R$-modules over a commutative ring $R$. A pairing is an $R$-linear map $Votimes Wto L$. An adjoint of an endomorphism $f:Vto V$ w.r.t a pairing $Votimes Woverset{g}{to}L$ is an endomorphism $f^{dagger ^g}:Wto W$ such that $g(fotimes 1)=g(1otimes f^{dagger^g})$.
When do such things exist and when are they unique? That is, what assumptions are needed on the modules involved, the pairing $g$, and $f$ itself? What if we suppose $g$ is a perfect pairing? (Perhaps this questions is as easily answerable for nice monoidal categories.)
For instance the adjugate of a linear endomorphism can be defined as an adjoint with respect to a canonical pairing. I would simply like to understand which "dualizability" assumptions are needed and where.
linear-algebra commutative-algebra category-theory adjoint-operators monoidal-categories
$endgroup$
add a comment |
$begingroup$
Let $V,W,L$ be $R$-modules over a commutative ring $R$. A pairing is an $R$-linear map $Votimes Wto L$. An adjoint of an endomorphism $f:Vto V$ w.r.t a pairing $Votimes Woverset{g}{to}L$ is an endomorphism $f^{dagger ^g}:Wto W$ such that $g(fotimes 1)=g(1otimes f^{dagger^g})$.
When do such things exist and when are they unique? That is, what assumptions are needed on the modules involved, the pairing $g$, and $f$ itself? What if we suppose $g$ is a perfect pairing? (Perhaps this questions is as easily answerable for nice monoidal categories.)
For instance the adjugate of a linear endomorphism can be defined as an adjoint with respect to a canonical pairing. I would simply like to understand which "dualizability" assumptions are needed and where.
linear-algebra commutative-algebra category-theory adjoint-operators monoidal-categories
$endgroup$
Let $V,W,L$ be $R$-modules over a commutative ring $R$. A pairing is an $R$-linear map $Votimes Wto L$. An adjoint of an endomorphism $f:Vto V$ w.r.t a pairing $Votimes Woverset{g}{to}L$ is an endomorphism $f^{dagger ^g}:Wto W$ such that $g(fotimes 1)=g(1otimes f^{dagger^g})$.
When do such things exist and when are they unique? That is, what assumptions are needed on the modules involved, the pairing $g$, and $f$ itself? What if we suppose $g$ is a perfect pairing? (Perhaps this questions is as easily answerable for nice monoidal categories.)
For instance the adjugate of a linear endomorphism can be defined as an adjoint with respect to a canonical pairing. I would simply like to understand which "dualizability" assumptions are needed and where.
linear-algebra commutative-algebra category-theory adjoint-operators monoidal-categories
linear-algebra commutative-algebra category-theory adjoint-operators monoidal-categories
edited Dec 24 '18 at 23:33
Arrow
asked Dec 24 '18 at 23:04
ArrowArrow
5,18431446
5,18431446
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
I'm going to denote adjoints by $*$ rather than $dagger_g$, for notational simplicity.
First answer:
Yes if $g$ is a perfect pairing, then adjoints always exist and are unique.
Let's exploit the tensor-hom adjunction and let $g_V: Wto newcommandHom{operatorname{Hom}}Hom(V,L)$ be the obvious map. Now for any $f$, we can consider the map $g_f : Wto Hom(V,L)$ defined by $g_f(w) = g_V(w)circ f$. Then in order
for an adjoint to exist, we must be able to solve the equation
$$ g_Vf^* = g_f.$$
Therefore if $g_V$ is an isomorphism (i.e. if $g$ is a perfect pairing), there is a unique $f^*$ satisfying the equation, $f^*=g_V^{-1}g_f$.
Second answer:
Let's generalize slightly. When can we solve $g_Vf^*=g_f$?
Consider the following diagram
$$require{AMScd}
begin{CD}
W @>g_f>> Hom(V,L) \
@Vf^*VV @| \
W @>>g_V> Hom(V,L)
end{CD}
$$
Well, one answer to when we can find such a $f^*$ is if $g_V$ is surjective and $W$ is projective. In this case $f^*$ won't be unique. In fact, this is roughly the most general we can get, though.
To generalize this slightly, observe that the image of $g_f$ had better be a subset of the image of $g_V$, otherwise there is no way we can solve it. However if the image of $g_f$ is a subset of the image of $g_V$, then we can replace $Hom(V,L)$ with $newcommandim{operatorname{im}}im g_V$, so that $g_V$ is now surjective to its image. Then as long as $W$ is projective, we can lift $g_f$ along $g_V$ to find $f^*$.
My final version of the second answer:
As long as $W$ is projective, and for every $w$, there exists $w'$ so that $g_V(w)circ f = g_V(w')$, then there exists a (possibly not unique) "adjoint" $f^*$ solving $g_Vf^* = g_f$.
$endgroup$
$begingroup$
So simple, so easy. Thanks!
$endgroup$
– Arrow
Dec 24 '18 at 23:36
$begingroup$
@Arrow in the case of perfect pairings, yes, I'm currently editing, since I think I should be able to make this slightly more general.
$endgroup$
– jgon
Dec 24 '18 at 23:37
$begingroup$
@Arrow, I generalized slightly.
$endgroup$
– jgon
Dec 24 '18 at 23:49
$begingroup$
Very cool. Thank you!
$endgroup$
– Arrow
Dec 24 '18 at 23:50
add a comment |
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1 Answer
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1 Answer
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$begingroup$
I'm going to denote adjoints by $*$ rather than $dagger_g$, for notational simplicity.
First answer:
Yes if $g$ is a perfect pairing, then adjoints always exist and are unique.
Let's exploit the tensor-hom adjunction and let $g_V: Wto newcommandHom{operatorname{Hom}}Hom(V,L)$ be the obvious map. Now for any $f$, we can consider the map $g_f : Wto Hom(V,L)$ defined by $g_f(w) = g_V(w)circ f$. Then in order
for an adjoint to exist, we must be able to solve the equation
$$ g_Vf^* = g_f.$$
Therefore if $g_V$ is an isomorphism (i.e. if $g$ is a perfect pairing), there is a unique $f^*$ satisfying the equation, $f^*=g_V^{-1}g_f$.
Second answer:
Let's generalize slightly. When can we solve $g_Vf^*=g_f$?
Consider the following diagram
$$require{AMScd}
begin{CD}
W @>g_f>> Hom(V,L) \
@Vf^*VV @| \
W @>>g_V> Hom(V,L)
end{CD}
$$
Well, one answer to when we can find such a $f^*$ is if $g_V$ is surjective and $W$ is projective. In this case $f^*$ won't be unique. In fact, this is roughly the most general we can get, though.
To generalize this slightly, observe that the image of $g_f$ had better be a subset of the image of $g_V$, otherwise there is no way we can solve it. However if the image of $g_f$ is a subset of the image of $g_V$, then we can replace $Hom(V,L)$ with $newcommandim{operatorname{im}}im g_V$, so that $g_V$ is now surjective to its image. Then as long as $W$ is projective, we can lift $g_f$ along $g_V$ to find $f^*$.
My final version of the second answer:
As long as $W$ is projective, and for every $w$, there exists $w'$ so that $g_V(w)circ f = g_V(w')$, then there exists a (possibly not unique) "adjoint" $f^*$ solving $g_Vf^* = g_f$.
$endgroup$
$begingroup$
So simple, so easy. Thanks!
$endgroup$
– Arrow
Dec 24 '18 at 23:36
$begingroup$
@Arrow in the case of perfect pairings, yes, I'm currently editing, since I think I should be able to make this slightly more general.
$endgroup$
– jgon
Dec 24 '18 at 23:37
$begingroup$
@Arrow, I generalized slightly.
$endgroup$
– jgon
Dec 24 '18 at 23:49
$begingroup$
Very cool. Thank you!
$endgroup$
– Arrow
Dec 24 '18 at 23:50
add a comment |
$begingroup$
I'm going to denote adjoints by $*$ rather than $dagger_g$, for notational simplicity.
First answer:
Yes if $g$ is a perfect pairing, then adjoints always exist and are unique.
Let's exploit the tensor-hom adjunction and let $g_V: Wto newcommandHom{operatorname{Hom}}Hom(V,L)$ be the obvious map. Now for any $f$, we can consider the map $g_f : Wto Hom(V,L)$ defined by $g_f(w) = g_V(w)circ f$. Then in order
for an adjoint to exist, we must be able to solve the equation
$$ g_Vf^* = g_f.$$
Therefore if $g_V$ is an isomorphism (i.e. if $g$ is a perfect pairing), there is a unique $f^*$ satisfying the equation, $f^*=g_V^{-1}g_f$.
Second answer:
Let's generalize slightly. When can we solve $g_Vf^*=g_f$?
Consider the following diagram
$$require{AMScd}
begin{CD}
W @>g_f>> Hom(V,L) \
@Vf^*VV @| \
W @>>g_V> Hom(V,L)
end{CD}
$$
Well, one answer to when we can find such a $f^*$ is if $g_V$ is surjective and $W$ is projective. In this case $f^*$ won't be unique. In fact, this is roughly the most general we can get, though.
To generalize this slightly, observe that the image of $g_f$ had better be a subset of the image of $g_V$, otherwise there is no way we can solve it. However if the image of $g_f$ is a subset of the image of $g_V$, then we can replace $Hom(V,L)$ with $newcommandim{operatorname{im}}im g_V$, so that $g_V$ is now surjective to its image. Then as long as $W$ is projective, we can lift $g_f$ along $g_V$ to find $f^*$.
My final version of the second answer:
As long as $W$ is projective, and for every $w$, there exists $w'$ so that $g_V(w)circ f = g_V(w')$, then there exists a (possibly not unique) "adjoint" $f^*$ solving $g_Vf^* = g_f$.
$endgroup$
$begingroup$
So simple, so easy. Thanks!
$endgroup$
– Arrow
Dec 24 '18 at 23:36
$begingroup$
@Arrow in the case of perfect pairings, yes, I'm currently editing, since I think I should be able to make this slightly more general.
$endgroup$
– jgon
Dec 24 '18 at 23:37
$begingroup$
@Arrow, I generalized slightly.
$endgroup$
– jgon
Dec 24 '18 at 23:49
$begingroup$
Very cool. Thank you!
$endgroup$
– Arrow
Dec 24 '18 at 23:50
add a comment |
$begingroup$
I'm going to denote adjoints by $*$ rather than $dagger_g$, for notational simplicity.
First answer:
Yes if $g$ is a perfect pairing, then adjoints always exist and are unique.
Let's exploit the tensor-hom adjunction and let $g_V: Wto newcommandHom{operatorname{Hom}}Hom(V,L)$ be the obvious map. Now for any $f$, we can consider the map $g_f : Wto Hom(V,L)$ defined by $g_f(w) = g_V(w)circ f$. Then in order
for an adjoint to exist, we must be able to solve the equation
$$ g_Vf^* = g_f.$$
Therefore if $g_V$ is an isomorphism (i.e. if $g$ is a perfect pairing), there is a unique $f^*$ satisfying the equation, $f^*=g_V^{-1}g_f$.
Second answer:
Let's generalize slightly. When can we solve $g_Vf^*=g_f$?
Consider the following diagram
$$require{AMScd}
begin{CD}
W @>g_f>> Hom(V,L) \
@Vf^*VV @| \
W @>>g_V> Hom(V,L)
end{CD}
$$
Well, one answer to when we can find such a $f^*$ is if $g_V$ is surjective and $W$ is projective. In this case $f^*$ won't be unique. In fact, this is roughly the most general we can get, though.
To generalize this slightly, observe that the image of $g_f$ had better be a subset of the image of $g_V$, otherwise there is no way we can solve it. However if the image of $g_f$ is a subset of the image of $g_V$, then we can replace $Hom(V,L)$ with $newcommandim{operatorname{im}}im g_V$, so that $g_V$ is now surjective to its image. Then as long as $W$ is projective, we can lift $g_f$ along $g_V$ to find $f^*$.
My final version of the second answer:
As long as $W$ is projective, and for every $w$, there exists $w'$ so that $g_V(w)circ f = g_V(w')$, then there exists a (possibly not unique) "adjoint" $f^*$ solving $g_Vf^* = g_f$.
$endgroup$
I'm going to denote adjoints by $*$ rather than $dagger_g$, for notational simplicity.
First answer:
Yes if $g$ is a perfect pairing, then adjoints always exist and are unique.
Let's exploit the tensor-hom adjunction and let $g_V: Wto newcommandHom{operatorname{Hom}}Hom(V,L)$ be the obvious map. Now for any $f$, we can consider the map $g_f : Wto Hom(V,L)$ defined by $g_f(w) = g_V(w)circ f$. Then in order
for an adjoint to exist, we must be able to solve the equation
$$ g_Vf^* = g_f.$$
Therefore if $g_V$ is an isomorphism (i.e. if $g$ is a perfect pairing), there is a unique $f^*$ satisfying the equation, $f^*=g_V^{-1}g_f$.
Second answer:
Let's generalize slightly. When can we solve $g_Vf^*=g_f$?
Consider the following diagram
$$require{AMScd}
begin{CD}
W @>g_f>> Hom(V,L) \
@Vf^*VV @| \
W @>>g_V> Hom(V,L)
end{CD}
$$
Well, one answer to when we can find such a $f^*$ is if $g_V$ is surjective and $W$ is projective. In this case $f^*$ won't be unique. In fact, this is roughly the most general we can get, though.
To generalize this slightly, observe that the image of $g_f$ had better be a subset of the image of $g_V$, otherwise there is no way we can solve it. However if the image of $g_f$ is a subset of the image of $g_V$, then we can replace $Hom(V,L)$ with $newcommandim{operatorname{im}}im g_V$, so that $g_V$ is now surjective to its image. Then as long as $W$ is projective, we can lift $g_f$ along $g_V$ to find $f^*$.
My final version of the second answer:
As long as $W$ is projective, and for every $w$, there exists $w'$ so that $g_V(w)circ f = g_V(w')$, then there exists a (possibly not unique) "adjoint" $f^*$ solving $g_Vf^* = g_f$.
edited Dec 24 '18 at 23:49
answered Dec 24 '18 at 23:35
jgonjgon
15.1k32042
15.1k32042
$begingroup$
So simple, so easy. Thanks!
$endgroup$
– Arrow
Dec 24 '18 at 23:36
$begingroup$
@Arrow in the case of perfect pairings, yes, I'm currently editing, since I think I should be able to make this slightly more general.
$endgroup$
– jgon
Dec 24 '18 at 23:37
$begingroup$
@Arrow, I generalized slightly.
$endgroup$
– jgon
Dec 24 '18 at 23:49
$begingroup$
Very cool. Thank you!
$endgroup$
– Arrow
Dec 24 '18 at 23:50
add a comment |
$begingroup$
So simple, so easy. Thanks!
$endgroup$
– Arrow
Dec 24 '18 at 23:36
$begingroup$
@Arrow in the case of perfect pairings, yes, I'm currently editing, since I think I should be able to make this slightly more general.
$endgroup$
– jgon
Dec 24 '18 at 23:37
$begingroup$
@Arrow, I generalized slightly.
$endgroup$
– jgon
Dec 24 '18 at 23:49
$begingroup$
Very cool. Thank you!
$endgroup$
– Arrow
Dec 24 '18 at 23:50
$begingroup$
So simple, so easy. Thanks!
$endgroup$
– Arrow
Dec 24 '18 at 23:36
$begingroup$
So simple, so easy. Thanks!
$endgroup$
– Arrow
Dec 24 '18 at 23:36
$begingroup$
@Arrow in the case of perfect pairings, yes, I'm currently editing, since I think I should be able to make this slightly more general.
$endgroup$
– jgon
Dec 24 '18 at 23:37
$begingroup$
@Arrow in the case of perfect pairings, yes, I'm currently editing, since I think I should be able to make this slightly more general.
$endgroup$
– jgon
Dec 24 '18 at 23:37
$begingroup$
@Arrow, I generalized slightly.
$endgroup$
– jgon
Dec 24 '18 at 23:49
$begingroup$
@Arrow, I generalized slightly.
$endgroup$
– jgon
Dec 24 '18 at 23:49
$begingroup$
Very cool. Thank you!
$endgroup$
– Arrow
Dec 24 '18 at 23:50
$begingroup$
Very cool. Thank you!
$endgroup$
– Arrow
Dec 24 '18 at 23:50
add a comment |
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