Tensor products of Hilbert spaces and Hilbert-Schmidt operators

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I am trying to get a better understanding of tensor products of Hilbert spaces. Suppose that $x,yinmathbb H$, where $mathbb H$ is a Hilbert space. As far as I understand, we can think about the tensor $xotimes y$ as a bounded linear operator from $mathbb H$ to $mathbb H$ defined by
$$
(xotimes y)(z)=langle z,yrangle x
$$
for each $xinmathbb H$ and the tensor product $mathbb Hotimes mathbb H$ as the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ (see here). Let us denote the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ by $HS(mathbb H)$.



It seems that we can think about $xotimes y$ as an element in a Hilbert space $HS(mathbb H)$ and we can consider a tensor $xotimes ytildeotimes zotimes w$ with $x,y,z,winmathbb H$ as an element in the tensor product $HS(mathbb H)tildeotimes HS(mathbb H)$.




Does it make sense to define the tensor $xotimes ytildeotimes zotimes w$ by setting
$$
(xotimes ytildeotimes zotimes w)(varphi)=langlevarphi,zotimes wrangle_mathrmHS(xotimes y)
$$
for each $varphiin HS(mathbb H)$, where $langlecdot,cdotrangle$ is a Hilbert-Schmidt inner product? This would mean that we can think about $mathbb Hotimes mathbb Htildeotimes mathbb Hotimes mathbb H$ as the space $HS(HS(mathbb H))$ and, if $varphi=uotimes v$ for some $u,v
inmathbb H$, we would have
$$
(xotimes ytildeotimes zotimes w)(uotimes v)
=langle uotimes v,zotimes wrangle_mathrmHS (xotimes y)
=langle u,zranglelangle w,vrangle(xotimes y).
$$




References (textbooks, papers, etc.) are very welcome. Any help is much appreciated!







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  • I've never read a construction like this about tensor products between Hilbert Spaces, nonetheless, if your first characterization of tensor products is fine, there wouldn't be any reason to think that your characterizaction about tensor product between tensor products would be bad. Also, if you found some references about this I would appreciate if you could comment about them.
    – Esteban Gutiérrez
    May 16 at 3:08














up vote
2
down vote

favorite












I am trying to get a better understanding of tensor products of Hilbert spaces. Suppose that $x,yinmathbb H$, where $mathbb H$ is a Hilbert space. As far as I understand, we can think about the tensor $xotimes y$ as a bounded linear operator from $mathbb H$ to $mathbb H$ defined by
$$
(xotimes y)(z)=langle z,yrangle x
$$
for each $xinmathbb H$ and the tensor product $mathbb Hotimes mathbb H$ as the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ (see here). Let us denote the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ by $HS(mathbb H)$.



It seems that we can think about $xotimes y$ as an element in a Hilbert space $HS(mathbb H)$ and we can consider a tensor $xotimes ytildeotimes zotimes w$ with $x,y,z,winmathbb H$ as an element in the tensor product $HS(mathbb H)tildeotimes HS(mathbb H)$.




Does it make sense to define the tensor $xotimes ytildeotimes zotimes w$ by setting
$$
(xotimes ytildeotimes zotimes w)(varphi)=langlevarphi,zotimes wrangle_mathrmHS(xotimes y)
$$
for each $varphiin HS(mathbb H)$, where $langlecdot,cdotrangle$ is a Hilbert-Schmidt inner product? This would mean that we can think about $mathbb Hotimes mathbb Htildeotimes mathbb Hotimes mathbb H$ as the space $HS(HS(mathbb H))$ and, if $varphi=uotimes v$ for some $u,v
inmathbb H$, we would have
$$
(xotimes ytildeotimes zotimes w)(uotimes v)
=langle uotimes v,zotimes wrangle_mathrmHS (xotimes y)
=langle u,zranglelangle w,vrangle(xotimes y).
$$




References (textbooks, papers, etc.) are very welcome. Any help is much appreciated!







share|cite|improve this question






















  • I've never read a construction like this about tensor products between Hilbert Spaces, nonetheless, if your first characterization of tensor products is fine, there wouldn't be any reason to think that your characterizaction about tensor product between tensor products would be bad. Also, if you found some references about this I would appreciate if you could comment about them.
    – Esteban Gutiérrez
    May 16 at 3:08












up vote
2
down vote

favorite









up vote
2
down vote

favorite











I am trying to get a better understanding of tensor products of Hilbert spaces. Suppose that $x,yinmathbb H$, where $mathbb H$ is a Hilbert space. As far as I understand, we can think about the tensor $xotimes y$ as a bounded linear operator from $mathbb H$ to $mathbb H$ defined by
$$
(xotimes y)(z)=langle z,yrangle x
$$
for each $xinmathbb H$ and the tensor product $mathbb Hotimes mathbb H$ as the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ (see here). Let us denote the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ by $HS(mathbb H)$.



It seems that we can think about $xotimes y$ as an element in a Hilbert space $HS(mathbb H)$ and we can consider a tensor $xotimes ytildeotimes zotimes w$ with $x,y,z,winmathbb H$ as an element in the tensor product $HS(mathbb H)tildeotimes HS(mathbb H)$.




Does it make sense to define the tensor $xotimes ytildeotimes zotimes w$ by setting
$$
(xotimes ytildeotimes zotimes w)(varphi)=langlevarphi,zotimes wrangle_mathrmHS(xotimes y)
$$
for each $varphiin HS(mathbb H)$, where $langlecdot,cdotrangle$ is a Hilbert-Schmidt inner product? This would mean that we can think about $mathbb Hotimes mathbb Htildeotimes mathbb Hotimes mathbb H$ as the space $HS(HS(mathbb H))$ and, if $varphi=uotimes v$ for some $u,v
inmathbb H$, we would have
$$
(xotimes ytildeotimes zotimes w)(uotimes v)
=langle uotimes v,zotimes wrangle_mathrmHS (xotimes y)
=langle u,zranglelangle w,vrangle(xotimes y).
$$




References (textbooks, papers, etc.) are very welcome. Any help is much appreciated!







share|cite|improve this question














I am trying to get a better understanding of tensor products of Hilbert spaces. Suppose that $x,yinmathbb H$, where $mathbb H$ is a Hilbert space. As far as I understand, we can think about the tensor $xotimes y$ as a bounded linear operator from $mathbb H$ to $mathbb H$ defined by
$$
(xotimes y)(z)=langle z,yrangle x
$$
for each $xinmathbb H$ and the tensor product $mathbb Hotimes mathbb H$ as the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ (see here). Let us denote the space of Hilbert-Schmidt operators from $mathbb H$ to $mathbb H$ by $HS(mathbb H)$.



It seems that we can think about $xotimes y$ as an element in a Hilbert space $HS(mathbb H)$ and we can consider a tensor $xotimes ytildeotimes zotimes w$ with $x,y,z,winmathbb H$ as an element in the tensor product $HS(mathbb H)tildeotimes HS(mathbb H)$.




Does it make sense to define the tensor $xotimes ytildeotimes zotimes w$ by setting
$$
(xotimes ytildeotimes zotimes w)(varphi)=langlevarphi,zotimes wrangle_mathrmHS(xotimes y)
$$
for each $varphiin HS(mathbb H)$, where $langlecdot,cdotrangle$ is a Hilbert-Schmidt inner product? This would mean that we can think about $mathbb Hotimes mathbb Htildeotimes mathbb Hotimes mathbb H$ as the space $HS(HS(mathbb H))$ and, if $varphi=uotimes v$ for some $u,v
inmathbb H$, we would have
$$
(xotimes ytildeotimes zotimes w)(uotimes v)
=langle uotimes v,zotimes wrangle_mathrmHS (xotimes y)
=langle u,zranglelangle w,vrangle(xotimes y).
$$




References (textbooks, papers, etc.) are very welcome. Any help is much appreciated!









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edited Mar 6 at 17:32

























asked Mar 5 at 10:39









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  • I've never read a construction like this about tensor products between Hilbert Spaces, nonetheless, if your first characterization of tensor products is fine, there wouldn't be any reason to think that your characterizaction about tensor product between tensor products would be bad. Also, if you found some references about this I would appreciate if you could comment about them.
    – Esteban Gutiérrez
    May 16 at 3:08
















  • I've never read a construction like this about tensor products between Hilbert Spaces, nonetheless, if your first characterization of tensor products is fine, there wouldn't be any reason to think that your characterizaction about tensor product between tensor products would be bad. Also, if you found some references about this I would appreciate if you could comment about them.
    – Esteban Gutiérrez
    May 16 at 3:08















I've never read a construction like this about tensor products between Hilbert Spaces, nonetheless, if your first characterization of tensor products is fine, there wouldn't be any reason to think that your characterizaction about tensor product between tensor products would be bad. Also, if you found some references about this I would appreciate if you could comment about them.
– Esteban Gutiérrez
May 16 at 3:08




I've never read a construction like this about tensor products between Hilbert Spaces, nonetheless, if your first characterization of tensor products is fine, there wouldn't be any reason to think that your characterizaction about tensor product between tensor products would be bad. Also, if you found some references about this I would appreciate if you could comment about them.
– Esteban Gutiérrez
May 16 at 3:08















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