$Hom(mathbb C^*, mathbb C^*)$?
$begingroup$
I want to calculate this group.
So far I noticed that if $h:mathbb C^* to mathbb C^*$ is a hoomorphism, then $h(z)=h(1cdot z)=h(1)cdot h(z)$ for any $zin mathbb C^*$. Thus $h(1)=1$.
Further I know that for any $tin mathbb C$ $h_t: zmapsto z^t$ satisfies $h(xy)=(xy)^t=x^ty^t=h(x)h(y)$. So this is an homomorphism, hence $Hom (mathbb C^*, mathbb C^*)subset mathbb C.$
But is this an equality, or are there more homomorphisms?
abstract-algebra group-theory
$endgroup$
add a comment |
$begingroup$
I want to calculate this group.
So far I noticed that if $h:mathbb C^* to mathbb C^*$ is a hoomorphism, then $h(z)=h(1cdot z)=h(1)cdot h(z)$ for any $zin mathbb C^*$. Thus $h(1)=1$.
Further I know that for any $tin mathbb C$ $h_t: zmapsto z^t$ satisfies $h(xy)=(xy)^t=x^ty^t=h(x)h(y)$. So this is an homomorphism, hence $Hom (mathbb C^*, mathbb C^*)subset mathbb C.$
But is this an equality, or are there more homomorphisms?
abstract-algebra group-theory
$endgroup$
$begingroup$
$z^t$ is not well defined if $t notin Bbb Z$.
$endgroup$
– Nicolas Hemelsoet
Nov 24 '18 at 11:11
$begingroup$
If $tnotinBbb Z$, then $zmapsto z^t$ (whatever that is) is not a homomorphism. If you believe AC then there are lots of homomorphisms.
$endgroup$
– Lord Shark the Unknown
Nov 24 '18 at 11:11
$begingroup$
Complex conjugation is one additional homomorphism you can write down (and of of course all composition of it with the homomorphisms you already mentioned)
$endgroup$
– Severin Schraven
Nov 24 '18 at 11:20
add a comment |
$begingroup$
I want to calculate this group.
So far I noticed that if $h:mathbb C^* to mathbb C^*$ is a hoomorphism, then $h(z)=h(1cdot z)=h(1)cdot h(z)$ for any $zin mathbb C^*$. Thus $h(1)=1$.
Further I know that for any $tin mathbb C$ $h_t: zmapsto z^t$ satisfies $h(xy)=(xy)^t=x^ty^t=h(x)h(y)$. So this is an homomorphism, hence $Hom (mathbb C^*, mathbb C^*)subset mathbb C.$
But is this an equality, or are there more homomorphisms?
abstract-algebra group-theory
$endgroup$
I want to calculate this group.
So far I noticed that if $h:mathbb C^* to mathbb C^*$ is a hoomorphism, then $h(z)=h(1cdot z)=h(1)cdot h(z)$ for any $zin mathbb C^*$. Thus $h(1)=1$.
Further I know that for any $tin mathbb C$ $h_t: zmapsto z^t$ satisfies $h(xy)=(xy)^t=x^ty^t=h(x)h(y)$. So this is an homomorphism, hence $Hom (mathbb C^*, mathbb C^*)subset mathbb C.$
But is this an equality, or are there more homomorphisms?
abstract-algebra group-theory
abstract-algebra group-theory
asked Nov 24 '18 at 11:09
J.DoeJ.Doe
214
214
$begingroup$
$z^t$ is not well defined if $t notin Bbb Z$.
$endgroup$
– Nicolas Hemelsoet
Nov 24 '18 at 11:11
$begingroup$
If $tnotinBbb Z$, then $zmapsto z^t$ (whatever that is) is not a homomorphism. If you believe AC then there are lots of homomorphisms.
$endgroup$
– Lord Shark the Unknown
Nov 24 '18 at 11:11
$begingroup$
Complex conjugation is one additional homomorphism you can write down (and of of course all composition of it with the homomorphisms you already mentioned)
$endgroup$
– Severin Schraven
Nov 24 '18 at 11:20
add a comment |
$begingroup$
$z^t$ is not well defined if $t notin Bbb Z$.
$endgroup$
– Nicolas Hemelsoet
Nov 24 '18 at 11:11
$begingroup$
If $tnotinBbb Z$, then $zmapsto z^t$ (whatever that is) is not a homomorphism. If you believe AC then there are lots of homomorphisms.
$endgroup$
– Lord Shark the Unknown
Nov 24 '18 at 11:11
$begingroup$
Complex conjugation is one additional homomorphism you can write down (and of of course all composition of it with the homomorphisms you already mentioned)
$endgroup$
– Severin Schraven
Nov 24 '18 at 11:20
$begingroup$
$z^t$ is not well defined if $t notin Bbb Z$.
$endgroup$
– Nicolas Hemelsoet
Nov 24 '18 at 11:11
$begingroup$
$z^t$ is not well defined if $t notin Bbb Z$.
$endgroup$
– Nicolas Hemelsoet
Nov 24 '18 at 11:11
$begingroup$
If $tnotinBbb Z$, then $zmapsto z^t$ (whatever that is) is not a homomorphism. If you believe AC then there are lots of homomorphisms.
$endgroup$
– Lord Shark the Unknown
Nov 24 '18 at 11:11
$begingroup$
If $tnotinBbb Z$, then $zmapsto z^t$ (whatever that is) is not a homomorphism. If you believe AC then there are lots of homomorphisms.
$endgroup$
– Lord Shark the Unknown
Nov 24 '18 at 11:11
$begingroup$
Complex conjugation is one additional homomorphism you can write down (and of of course all composition of it with the homomorphisms you already mentioned)
$endgroup$
– Severin Schraven
Nov 24 '18 at 11:20
$begingroup$
Complex conjugation is one additional homomorphism you can write down (and of of course all composition of it with the homomorphisms you already mentioned)
$endgroup$
– Severin Schraven
Nov 24 '18 at 11:20
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
One can classify simply the continuous homomorphisms from $Bbb C^*$
to itself. They are the maps
$$zmapsto z^nexp(aln|z|)$$
where $ninBbb Z$ and $ainBbb C$.
But assuming the axiom of choice, one proves that $Bbb C^*$
is isomorphic to the additive $(Bbb Q/Bbb Z)timesbigoplus_IBbb Q$
where $I$ is an uncountable index set. This group has $2^{2^{aleph_0}}$
automorphisms, an awful lot.
$endgroup$
$begingroup$
I see that the defined map is a continuous homomorphism. But how can we be sure that there are no others?
$endgroup$
– J.Doe
Nov 24 '18 at 13:23
add a comment |
$begingroup$
Multiplicative notation is not common in abelian group theory.
We can use the fact that $mathbb{C}^*congmathbb{R}oplusmathbb{T}$, where $mathbb{T}=mathbb{R}/mathbb{Z}$, via
$$
(r,u+mathbb{Z})mapsto e^r(cos(2pi u)+isin(2pi u))
$$
Thus the group of endomorphisms of $mathbb{C}^*$ is the direct sum of
$$DeclareMathOperator{Hom}{Hom}
Hom(mathbb{R},mathbb{R})oplusHom(mathbb{R},mathbb{T})oplus
Hom(mathbb{T},mathbb{T})oplusHom(mathbb{T},mathbb{R})
$$
The first two groups are big $mathbb{Q}$-vector spaces, the third one is a wild beast. If we consider the exact sequence $0tomathbb{Z}tomathbb{R}tomathbb{T}to0$, then we have
$$
0toHom(mathbb{T},mathbb{T})toHom(mathbb{R},mathbb{T})toHom(mathbb{Z},mathbb{T})to0
$$
and $Hom(mathbb{Z},mathbb{T})congmathbb{T}$. So $Hom(mathbb{T},mathbb{T})$ sits in a big $mathbb{Q}$-vector space so that the cokernel is $mathbb{T}$.
What's $Hom(mathbb{T},mathbb{R})$? A similar sequence arises:
$$
0toHom(mathbb{T},mathbb{R})toHom(mathbb{R},mathbb{R})toHom(mathbb{Z},mathbb{R})to0
$$
As you see, the group $Hom(mathbb{C}^*,mathbb{C}^*)$ is much bigger than $mathbb{C}$.
$endgroup$
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
One can classify simply the continuous homomorphisms from $Bbb C^*$
to itself. They are the maps
$$zmapsto z^nexp(aln|z|)$$
where $ninBbb Z$ and $ainBbb C$.
But assuming the axiom of choice, one proves that $Bbb C^*$
is isomorphic to the additive $(Bbb Q/Bbb Z)timesbigoplus_IBbb Q$
where $I$ is an uncountable index set. This group has $2^{2^{aleph_0}}$
automorphisms, an awful lot.
$endgroup$
$begingroup$
I see that the defined map is a continuous homomorphism. But how can we be sure that there are no others?
$endgroup$
– J.Doe
Nov 24 '18 at 13:23
add a comment |
$begingroup$
One can classify simply the continuous homomorphisms from $Bbb C^*$
to itself. They are the maps
$$zmapsto z^nexp(aln|z|)$$
where $ninBbb Z$ and $ainBbb C$.
But assuming the axiom of choice, one proves that $Bbb C^*$
is isomorphic to the additive $(Bbb Q/Bbb Z)timesbigoplus_IBbb Q$
where $I$ is an uncountable index set. This group has $2^{2^{aleph_0}}$
automorphisms, an awful lot.
$endgroup$
$begingroup$
I see that the defined map is a continuous homomorphism. But how can we be sure that there are no others?
$endgroup$
– J.Doe
Nov 24 '18 at 13:23
add a comment |
$begingroup$
One can classify simply the continuous homomorphisms from $Bbb C^*$
to itself. They are the maps
$$zmapsto z^nexp(aln|z|)$$
where $ninBbb Z$ and $ainBbb C$.
But assuming the axiom of choice, one proves that $Bbb C^*$
is isomorphic to the additive $(Bbb Q/Bbb Z)timesbigoplus_IBbb Q$
where $I$ is an uncountable index set. This group has $2^{2^{aleph_0}}$
automorphisms, an awful lot.
$endgroup$
One can classify simply the continuous homomorphisms from $Bbb C^*$
to itself. They are the maps
$$zmapsto z^nexp(aln|z|)$$
where $ninBbb Z$ and $ainBbb C$.
But assuming the axiom of choice, one proves that $Bbb C^*$
is isomorphic to the additive $(Bbb Q/Bbb Z)timesbigoplus_IBbb Q$
where $I$ is an uncountable index set. This group has $2^{2^{aleph_0}}$
automorphisms, an awful lot.
answered Nov 24 '18 at 11:25
Lord Shark the UnknownLord Shark the Unknown
102k960132
102k960132
$begingroup$
I see that the defined map is a continuous homomorphism. But how can we be sure that there are no others?
$endgroup$
– J.Doe
Nov 24 '18 at 13:23
add a comment |
$begingroup$
I see that the defined map is a continuous homomorphism. But how can we be sure that there are no others?
$endgroup$
– J.Doe
Nov 24 '18 at 13:23
$begingroup$
I see that the defined map is a continuous homomorphism. But how can we be sure that there are no others?
$endgroup$
– J.Doe
Nov 24 '18 at 13:23
$begingroup$
I see that the defined map is a continuous homomorphism. But how can we be sure that there are no others?
$endgroup$
– J.Doe
Nov 24 '18 at 13:23
add a comment |
$begingroup$
Multiplicative notation is not common in abelian group theory.
We can use the fact that $mathbb{C}^*congmathbb{R}oplusmathbb{T}$, where $mathbb{T}=mathbb{R}/mathbb{Z}$, via
$$
(r,u+mathbb{Z})mapsto e^r(cos(2pi u)+isin(2pi u))
$$
Thus the group of endomorphisms of $mathbb{C}^*$ is the direct sum of
$$DeclareMathOperator{Hom}{Hom}
Hom(mathbb{R},mathbb{R})oplusHom(mathbb{R},mathbb{T})oplus
Hom(mathbb{T},mathbb{T})oplusHom(mathbb{T},mathbb{R})
$$
The first two groups are big $mathbb{Q}$-vector spaces, the third one is a wild beast. If we consider the exact sequence $0tomathbb{Z}tomathbb{R}tomathbb{T}to0$, then we have
$$
0toHom(mathbb{T},mathbb{T})toHom(mathbb{R},mathbb{T})toHom(mathbb{Z},mathbb{T})to0
$$
and $Hom(mathbb{Z},mathbb{T})congmathbb{T}$. So $Hom(mathbb{T},mathbb{T})$ sits in a big $mathbb{Q}$-vector space so that the cokernel is $mathbb{T}$.
What's $Hom(mathbb{T},mathbb{R})$? A similar sequence arises:
$$
0toHom(mathbb{T},mathbb{R})toHom(mathbb{R},mathbb{R})toHom(mathbb{Z},mathbb{R})to0
$$
As you see, the group $Hom(mathbb{C}^*,mathbb{C}^*)$ is much bigger than $mathbb{C}$.
$endgroup$
add a comment |
$begingroup$
Multiplicative notation is not common in abelian group theory.
We can use the fact that $mathbb{C}^*congmathbb{R}oplusmathbb{T}$, where $mathbb{T}=mathbb{R}/mathbb{Z}$, via
$$
(r,u+mathbb{Z})mapsto e^r(cos(2pi u)+isin(2pi u))
$$
Thus the group of endomorphisms of $mathbb{C}^*$ is the direct sum of
$$DeclareMathOperator{Hom}{Hom}
Hom(mathbb{R},mathbb{R})oplusHom(mathbb{R},mathbb{T})oplus
Hom(mathbb{T},mathbb{T})oplusHom(mathbb{T},mathbb{R})
$$
The first two groups are big $mathbb{Q}$-vector spaces, the third one is a wild beast. If we consider the exact sequence $0tomathbb{Z}tomathbb{R}tomathbb{T}to0$, then we have
$$
0toHom(mathbb{T},mathbb{T})toHom(mathbb{R},mathbb{T})toHom(mathbb{Z},mathbb{T})to0
$$
and $Hom(mathbb{Z},mathbb{T})congmathbb{T}$. So $Hom(mathbb{T},mathbb{T})$ sits in a big $mathbb{Q}$-vector space so that the cokernel is $mathbb{T}$.
What's $Hom(mathbb{T},mathbb{R})$? A similar sequence arises:
$$
0toHom(mathbb{T},mathbb{R})toHom(mathbb{R},mathbb{R})toHom(mathbb{Z},mathbb{R})to0
$$
As you see, the group $Hom(mathbb{C}^*,mathbb{C}^*)$ is much bigger than $mathbb{C}$.
$endgroup$
add a comment |
$begingroup$
Multiplicative notation is not common in abelian group theory.
We can use the fact that $mathbb{C}^*congmathbb{R}oplusmathbb{T}$, where $mathbb{T}=mathbb{R}/mathbb{Z}$, via
$$
(r,u+mathbb{Z})mapsto e^r(cos(2pi u)+isin(2pi u))
$$
Thus the group of endomorphisms of $mathbb{C}^*$ is the direct sum of
$$DeclareMathOperator{Hom}{Hom}
Hom(mathbb{R},mathbb{R})oplusHom(mathbb{R},mathbb{T})oplus
Hom(mathbb{T},mathbb{T})oplusHom(mathbb{T},mathbb{R})
$$
The first two groups are big $mathbb{Q}$-vector spaces, the third one is a wild beast. If we consider the exact sequence $0tomathbb{Z}tomathbb{R}tomathbb{T}to0$, then we have
$$
0toHom(mathbb{T},mathbb{T})toHom(mathbb{R},mathbb{T})toHom(mathbb{Z},mathbb{T})to0
$$
and $Hom(mathbb{Z},mathbb{T})congmathbb{T}$. So $Hom(mathbb{T},mathbb{T})$ sits in a big $mathbb{Q}$-vector space so that the cokernel is $mathbb{T}$.
What's $Hom(mathbb{T},mathbb{R})$? A similar sequence arises:
$$
0toHom(mathbb{T},mathbb{R})toHom(mathbb{R},mathbb{R})toHom(mathbb{Z},mathbb{R})to0
$$
As you see, the group $Hom(mathbb{C}^*,mathbb{C}^*)$ is much bigger than $mathbb{C}$.
$endgroup$
Multiplicative notation is not common in abelian group theory.
We can use the fact that $mathbb{C}^*congmathbb{R}oplusmathbb{T}$, where $mathbb{T}=mathbb{R}/mathbb{Z}$, via
$$
(r,u+mathbb{Z})mapsto e^r(cos(2pi u)+isin(2pi u))
$$
Thus the group of endomorphisms of $mathbb{C}^*$ is the direct sum of
$$DeclareMathOperator{Hom}{Hom}
Hom(mathbb{R},mathbb{R})oplusHom(mathbb{R},mathbb{T})oplus
Hom(mathbb{T},mathbb{T})oplusHom(mathbb{T},mathbb{R})
$$
The first two groups are big $mathbb{Q}$-vector spaces, the third one is a wild beast. If we consider the exact sequence $0tomathbb{Z}tomathbb{R}tomathbb{T}to0$, then we have
$$
0toHom(mathbb{T},mathbb{T})toHom(mathbb{R},mathbb{T})toHom(mathbb{Z},mathbb{T})to0
$$
and $Hom(mathbb{Z},mathbb{T})congmathbb{T}$. So $Hom(mathbb{T},mathbb{T})$ sits in a big $mathbb{Q}$-vector space so that the cokernel is $mathbb{T}$.
What's $Hom(mathbb{T},mathbb{R})$? A similar sequence arises:
$$
0toHom(mathbb{T},mathbb{R})toHom(mathbb{R},mathbb{R})toHom(mathbb{Z},mathbb{R})to0
$$
As you see, the group $Hom(mathbb{C}^*,mathbb{C}^*)$ is much bigger than $mathbb{C}$.
edited Nov 24 '18 at 12:20
answered Nov 24 '18 at 12:15
egregegreg
180k1485202
180k1485202
add a comment |
add a comment |
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$begingroup$
$z^t$ is not well defined if $t notin Bbb Z$.
$endgroup$
– Nicolas Hemelsoet
Nov 24 '18 at 11:11
$begingroup$
If $tnotinBbb Z$, then $zmapsto z^t$ (whatever that is) is not a homomorphism. If you believe AC then there are lots of homomorphisms.
$endgroup$
– Lord Shark the Unknown
Nov 24 '18 at 11:11
$begingroup$
Complex conjugation is one additional homomorphism you can write down (and of of course all composition of it with the homomorphisms you already mentioned)
$endgroup$
– Severin Schraven
Nov 24 '18 at 11:20