Why $det_mathcal B_n(e_1,…,e_p,d_p+1,d_p+2+c_p+2,…,c_n+d_n)=0$?

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The aim of the exercise is to prove that $$detbeginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$$
where $Bin mathbb R^ptimes p$, $Cin mathbb R^qtimes q$, $n=p+q$ and $M:=beginpmatrixB&D\0&Cendpmatrixinmathbb R^ntimes n$.



Let $mathcal B_n(e_1,...,e_n)$ the canonical basis of $mathbb R^n$, $E=Spane_1,...,e_p$ and $E'=Spane_p+1,...,e_n$. Therefore $mathbb R^n=Eoplus E'$.



Set $M=(m_j)_1leq jleq n$ where $m_j$ are the column of $M$ and are defined as $m_j=b_j+0$ if $1leq jleq p$ and $m_j=d_j+c_j$ if $p+1leq jleq n$. Therefore



$$det(M)=det_mathcal B_n(m_1,...,m_n)=det_mathcal B_n(b_1,...,b_p,c_p+1+d_p+1,...,c_n+d_n).$$



Let $$alpha :E^pto mathbb R$$
defined as $$alpha (x_1,...,x_p)=det_mathcal B_n(x_1,...,x_p,c_p+1+d_p+1,...,c_n+d_n).$$
Recall that $e_1,...,e_p$ is a basis of $E$. Therefore, by linearity :
$$alpha (e_1,...,e_p)=det_mathcal B_n(e_1,...,e_p,c_p+1,...,c_n+d_n)+underbracedet_mathcal B_n(e_1,...,e_p,d_p+1,...,c_n+d_n)_=0,$$
but I don't understand why the last term is $0$.







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  • 1




    d_p+1 is a linear combination of e_i
    – Mike Hawk
    Aug 2 at 16:45










  • @MikeHawk: Thank you but how do you know that ?
    – Peter
    Aug 2 at 16:46










  • Petulant downvoting is not a great strategy. Certainly does not encourage one to help you.
    – copper.hat
    Aug 2 at 17:23















up vote
2
down vote

favorite
1












The aim of the exercise is to prove that $$detbeginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$$
where $Bin mathbb R^ptimes p$, $Cin mathbb R^qtimes q$, $n=p+q$ and $M:=beginpmatrixB&D\0&Cendpmatrixinmathbb R^ntimes n$.



Let $mathcal B_n(e_1,...,e_n)$ the canonical basis of $mathbb R^n$, $E=Spane_1,...,e_p$ and $E'=Spane_p+1,...,e_n$. Therefore $mathbb R^n=Eoplus E'$.



Set $M=(m_j)_1leq jleq n$ where $m_j$ are the column of $M$ and are defined as $m_j=b_j+0$ if $1leq jleq p$ and $m_j=d_j+c_j$ if $p+1leq jleq n$. Therefore



$$det(M)=det_mathcal B_n(m_1,...,m_n)=det_mathcal B_n(b_1,...,b_p,c_p+1+d_p+1,...,c_n+d_n).$$



Let $$alpha :E^pto mathbb R$$
defined as $$alpha (x_1,...,x_p)=det_mathcal B_n(x_1,...,x_p,c_p+1+d_p+1,...,c_n+d_n).$$
Recall that $e_1,...,e_p$ is a basis of $E$. Therefore, by linearity :
$$alpha (e_1,...,e_p)=det_mathcal B_n(e_1,...,e_p,c_p+1,...,c_n+d_n)+underbracedet_mathcal B_n(e_1,...,e_p,d_p+1,...,c_n+d_n)_=0,$$
but I don't understand why the last term is $0$.







share|cite|improve this question















  • 1




    d_p+1 is a linear combination of e_i
    – Mike Hawk
    Aug 2 at 16:45










  • @MikeHawk: Thank you but how do you know that ?
    – Peter
    Aug 2 at 16:46










  • Petulant downvoting is not a great strategy. Certainly does not encourage one to help you.
    – copper.hat
    Aug 2 at 17:23













up vote
2
down vote

favorite
1









up vote
2
down vote

favorite
1






1





The aim of the exercise is to prove that $$detbeginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$$
where $Bin mathbb R^ptimes p$, $Cin mathbb R^qtimes q$, $n=p+q$ and $M:=beginpmatrixB&D\0&Cendpmatrixinmathbb R^ntimes n$.



Let $mathcal B_n(e_1,...,e_n)$ the canonical basis of $mathbb R^n$, $E=Spane_1,...,e_p$ and $E'=Spane_p+1,...,e_n$. Therefore $mathbb R^n=Eoplus E'$.



Set $M=(m_j)_1leq jleq n$ where $m_j$ are the column of $M$ and are defined as $m_j=b_j+0$ if $1leq jleq p$ and $m_j=d_j+c_j$ if $p+1leq jleq n$. Therefore



$$det(M)=det_mathcal B_n(m_1,...,m_n)=det_mathcal B_n(b_1,...,b_p,c_p+1+d_p+1,...,c_n+d_n).$$



Let $$alpha :E^pto mathbb R$$
defined as $$alpha (x_1,...,x_p)=det_mathcal B_n(x_1,...,x_p,c_p+1+d_p+1,...,c_n+d_n).$$
Recall that $e_1,...,e_p$ is a basis of $E$. Therefore, by linearity :
$$alpha (e_1,...,e_p)=det_mathcal B_n(e_1,...,e_p,c_p+1,...,c_n+d_n)+underbracedet_mathcal B_n(e_1,...,e_p,d_p+1,...,c_n+d_n)_=0,$$
but I don't understand why the last term is $0$.







share|cite|improve this question











The aim of the exercise is to prove that $$detbeginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$$
where $Bin mathbb R^ptimes p$, $Cin mathbb R^qtimes q$, $n=p+q$ and $M:=beginpmatrixB&D\0&Cendpmatrixinmathbb R^ntimes n$.



Let $mathcal B_n(e_1,...,e_n)$ the canonical basis of $mathbb R^n$, $E=Spane_1,...,e_p$ and $E'=Spane_p+1,...,e_n$. Therefore $mathbb R^n=Eoplus E'$.



Set $M=(m_j)_1leq jleq n$ where $m_j$ are the column of $M$ and are defined as $m_j=b_j+0$ if $1leq jleq p$ and $m_j=d_j+c_j$ if $p+1leq jleq n$. Therefore



$$det(M)=det_mathcal B_n(m_1,...,m_n)=det_mathcal B_n(b_1,...,b_p,c_p+1+d_p+1,...,c_n+d_n).$$



Let $$alpha :E^pto mathbb R$$
defined as $$alpha (x_1,...,x_p)=det_mathcal B_n(x_1,...,x_p,c_p+1+d_p+1,...,c_n+d_n).$$
Recall that $e_1,...,e_p$ is a basis of $E$. Therefore, by linearity :
$$alpha (e_1,...,e_p)=det_mathcal B_n(e_1,...,e_p,c_p+1,...,c_n+d_n)+underbracedet_mathcal B_n(e_1,...,e_p,d_p+1,...,c_n+d_n)_=0,$$
but I don't understand why the last term is $0$.









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asked Aug 2 at 16:41









Peter

326112




326112







  • 1




    d_p+1 is a linear combination of e_i
    – Mike Hawk
    Aug 2 at 16:45










  • @MikeHawk: Thank you but how do you know that ?
    – Peter
    Aug 2 at 16:46










  • Petulant downvoting is not a great strategy. Certainly does not encourage one to help you.
    – copper.hat
    Aug 2 at 17:23













  • 1




    d_p+1 is a linear combination of e_i
    – Mike Hawk
    Aug 2 at 16:45










  • @MikeHawk: Thank you but how do you know that ?
    – Peter
    Aug 2 at 16:46










  • Petulant downvoting is not a great strategy. Certainly does not encourage one to help you.
    – copper.hat
    Aug 2 at 17:23








1




1




d_p+1 is a linear combination of e_i
– Mike Hawk
Aug 2 at 16:45




d_p+1 is a linear combination of e_i
– Mike Hawk
Aug 2 at 16:45












@MikeHawk: Thank you but how do you know that ?
– Peter
Aug 2 at 16:46




@MikeHawk: Thank you but how do you know that ?
– Peter
Aug 2 at 16:46












Petulant downvoting is not a great strategy. Certainly does not encourage one to help you.
– copper.hat
Aug 2 at 17:23





Petulant downvoting is not a great strategy. Certainly does not encourage one to help you.
– copper.hat
Aug 2 at 17:23











2 Answers
2






active

oldest

votes

















up vote
2
down vote













Here is another way:



Note that if $C$ is invertible we have
$beginbmatrix B & D \ 0 & Cendbmatrix = beginbmatrix I & DC^-1 \ 0 & Iendbmatrix beginbmatrix B & 0 \ 0 & Cendbmatrix$ and so we see that
$det beginbmatrix B & D \ 0 & Cendbmatrix = = det B det C$.



Since this is true for all invertible $C$, $det$ is continuous and the invertible matrices are dense we see that it is true for all $C$.






share|cite|improve this answer





















  • This doesn't answer my question. By the why, your hint is equivalent to prove that $det beginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$, so I don't really see in what it helps.
    – Peter
    Aug 2 at 17:15










  • Why the downvote?
    – copper.hat
    Aug 2 at 17:18






  • 1




    @Peter: The computation of $det$ for the first matrix is straightforward since there is exactly one permutation that needs to be examined. The computation for the second is also straightforward for similar reasons.
    – copper.hat
    Aug 2 at 17:22











  • I did downvoted (please don't take it against you). I'm not criticizing your mathematical skills that are very huge. The reason for my downvote it's that proving your hint is almost equivalent to prove what I asked (even if it's more straightforward, it's almost the same)
    – Peter
    Aug 2 at 22:54

















up vote
0
down vote













As Mike Hawk mentioned $d_p+1$ is a linear combination of $(e_i)_1le ile p$ since it is an element of $mathbb R^p$ which has as base $(e_i)_1le ile p$






share|cite|improve this answer





















  • First, you just copy a comment (that is not very well see). Then, how do you know that $d_p+1$ is a linear combinaison of $(e_i)_1leq ileq p$ ?
    – Peter
    Aug 2 at 17:14










  • Yes I know, I did that since I am new and I cannot comment, so it was my only solution. As I told you $(e_i)_ile p$ is a base of $mathbb R^p$. And $d_p+1$ is in $mathbb R^p$
    – Ahmed Lazhar
    Aug 2 at 17:25






  • 1




    +1: This is a perfectly good answer, I do not understand the downvote.
    – copper.hat
    Aug 2 at 17:25











  • @copper.hat: In fact I did downvote, but I forgot that less 50 reputation can't comment. I tried to cancel my downvote, but it doesn't work...
    – Peter
    Aug 2 at 22:35






  • 1




    Sorry for the downvote, I forgot the fact that you can't comment. And thank you for your answer.
    – Peter
    Aug 2 at 22:36










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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
2
down vote













Here is another way:



Note that if $C$ is invertible we have
$beginbmatrix B & D \ 0 & Cendbmatrix = beginbmatrix I & DC^-1 \ 0 & Iendbmatrix beginbmatrix B & 0 \ 0 & Cendbmatrix$ and so we see that
$det beginbmatrix B & D \ 0 & Cendbmatrix = = det B det C$.



Since this is true for all invertible $C$, $det$ is continuous and the invertible matrices are dense we see that it is true for all $C$.






share|cite|improve this answer





















  • This doesn't answer my question. By the why, your hint is equivalent to prove that $det beginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$, so I don't really see in what it helps.
    – Peter
    Aug 2 at 17:15










  • Why the downvote?
    – copper.hat
    Aug 2 at 17:18






  • 1




    @Peter: The computation of $det$ for the first matrix is straightforward since there is exactly one permutation that needs to be examined. The computation for the second is also straightforward for similar reasons.
    – copper.hat
    Aug 2 at 17:22











  • I did downvoted (please don't take it against you). I'm not criticizing your mathematical skills that are very huge. The reason for my downvote it's that proving your hint is almost equivalent to prove what I asked (even if it's more straightforward, it's almost the same)
    – Peter
    Aug 2 at 22:54














up vote
2
down vote













Here is another way:



Note that if $C$ is invertible we have
$beginbmatrix B & D \ 0 & Cendbmatrix = beginbmatrix I & DC^-1 \ 0 & Iendbmatrix beginbmatrix B & 0 \ 0 & Cendbmatrix$ and so we see that
$det beginbmatrix B & D \ 0 & Cendbmatrix = = det B det C$.



Since this is true for all invertible $C$, $det$ is continuous and the invertible matrices are dense we see that it is true for all $C$.






share|cite|improve this answer





















  • This doesn't answer my question. By the why, your hint is equivalent to prove that $det beginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$, so I don't really see in what it helps.
    – Peter
    Aug 2 at 17:15










  • Why the downvote?
    – copper.hat
    Aug 2 at 17:18






  • 1




    @Peter: The computation of $det$ for the first matrix is straightforward since there is exactly one permutation that needs to be examined. The computation for the second is also straightforward for similar reasons.
    – copper.hat
    Aug 2 at 17:22











  • I did downvoted (please don't take it against you). I'm not criticizing your mathematical skills that are very huge. The reason for my downvote it's that proving your hint is almost equivalent to prove what I asked (even if it's more straightforward, it's almost the same)
    – Peter
    Aug 2 at 22:54












up vote
2
down vote










up vote
2
down vote









Here is another way:



Note that if $C$ is invertible we have
$beginbmatrix B & D \ 0 & Cendbmatrix = beginbmatrix I & DC^-1 \ 0 & Iendbmatrix beginbmatrix B & 0 \ 0 & Cendbmatrix$ and so we see that
$det beginbmatrix B & D \ 0 & Cendbmatrix = = det B det C$.



Since this is true for all invertible $C$, $det$ is continuous and the invertible matrices are dense we see that it is true for all $C$.






share|cite|improve this answer













Here is another way:



Note that if $C$ is invertible we have
$beginbmatrix B & D \ 0 & Cendbmatrix = beginbmatrix I & DC^-1 \ 0 & Iendbmatrix beginbmatrix B & 0 \ 0 & Cendbmatrix$ and so we see that
$det beginbmatrix B & D \ 0 & Cendbmatrix = = det B det C$.



Since this is true for all invertible $C$, $det$ is continuous and the invertible matrices are dense we see that it is true for all $C$.







share|cite|improve this answer













share|cite|improve this answer



share|cite|improve this answer











answered Aug 2 at 17:02









copper.hat

122k557155




122k557155











  • This doesn't answer my question. By the why, your hint is equivalent to prove that $det beginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$, so I don't really see in what it helps.
    – Peter
    Aug 2 at 17:15










  • Why the downvote?
    – copper.hat
    Aug 2 at 17:18






  • 1




    @Peter: The computation of $det$ for the first matrix is straightforward since there is exactly one permutation that needs to be examined. The computation for the second is also straightforward for similar reasons.
    – copper.hat
    Aug 2 at 17:22











  • I did downvoted (please don't take it against you). I'm not criticizing your mathematical skills that are very huge. The reason for my downvote it's that proving your hint is almost equivalent to prove what I asked (even if it's more straightforward, it's almost the same)
    – Peter
    Aug 2 at 22:54
















  • This doesn't answer my question. By the why, your hint is equivalent to prove that $det beginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$, so I don't really see in what it helps.
    – Peter
    Aug 2 at 17:15










  • Why the downvote?
    – copper.hat
    Aug 2 at 17:18






  • 1




    @Peter: The computation of $det$ for the first matrix is straightforward since there is exactly one permutation that needs to be examined. The computation for the second is also straightforward for similar reasons.
    – copper.hat
    Aug 2 at 17:22











  • I did downvoted (please don't take it against you). I'm not criticizing your mathematical skills that are very huge. The reason for my downvote it's that proving your hint is almost equivalent to prove what I asked (even if it's more straightforward, it's almost the same)
    – Peter
    Aug 2 at 22:54















This doesn't answer my question. By the why, your hint is equivalent to prove that $det beginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$, so I don't really see in what it helps.
– Peter
Aug 2 at 17:15




This doesn't answer my question. By the why, your hint is equivalent to prove that $det beginpmatrixB&D\0&Cendpmatrix=det(B)det(C)$, so I don't really see in what it helps.
– Peter
Aug 2 at 17:15












Why the downvote?
– copper.hat
Aug 2 at 17:18




Why the downvote?
– copper.hat
Aug 2 at 17:18




1




1




@Peter: The computation of $det$ for the first matrix is straightforward since there is exactly one permutation that needs to be examined. The computation for the second is also straightforward for similar reasons.
– copper.hat
Aug 2 at 17:22





@Peter: The computation of $det$ for the first matrix is straightforward since there is exactly one permutation that needs to be examined. The computation for the second is also straightforward for similar reasons.
– copper.hat
Aug 2 at 17:22













I did downvoted (please don't take it against you). I'm not criticizing your mathematical skills that are very huge. The reason for my downvote it's that proving your hint is almost equivalent to prove what I asked (even if it's more straightforward, it's almost the same)
– Peter
Aug 2 at 22:54




I did downvoted (please don't take it against you). I'm not criticizing your mathematical skills that are very huge. The reason for my downvote it's that proving your hint is almost equivalent to prove what I asked (even if it's more straightforward, it's almost the same)
– Peter
Aug 2 at 22:54










up vote
0
down vote













As Mike Hawk mentioned $d_p+1$ is a linear combination of $(e_i)_1le ile p$ since it is an element of $mathbb R^p$ which has as base $(e_i)_1le ile p$






share|cite|improve this answer





















  • First, you just copy a comment (that is not very well see). Then, how do you know that $d_p+1$ is a linear combinaison of $(e_i)_1leq ileq p$ ?
    – Peter
    Aug 2 at 17:14










  • Yes I know, I did that since I am new and I cannot comment, so it was my only solution. As I told you $(e_i)_ile p$ is a base of $mathbb R^p$. And $d_p+1$ is in $mathbb R^p$
    – Ahmed Lazhar
    Aug 2 at 17:25






  • 1




    +1: This is a perfectly good answer, I do not understand the downvote.
    – copper.hat
    Aug 2 at 17:25











  • @copper.hat: In fact I did downvote, but I forgot that less 50 reputation can't comment. I tried to cancel my downvote, but it doesn't work...
    – Peter
    Aug 2 at 22:35






  • 1




    Sorry for the downvote, I forgot the fact that you can't comment. And thank you for your answer.
    – Peter
    Aug 2 at 22:36














up vote
0
down vote













As Mike Hawk mentioned $d_p+1$ is a linear combination of $(e_i)_1le ile p$ since it is an element of $mathbb R^p$ which has as base $(e_i)_1le ile p$






share|cite|improve this answer





















  • First, you just copy a comment (that is not very well see). Then, how do you know that $d_p+1$ is a linear combinaison of $(e_i)_1leq ileq p$ ?
    – Peter
    Aug 2 at 17:14










  • Yes I know, I did that since I am new and I cannot comment, so it was my only solution. As I told you $(e_i)_ile p$ is a base of $mathbb R^p$. And $d_p+1$ is in $mathbb R^p$
    – Ahmed Lazhar
    Aug 2 at 17:25






  • 1




    +1: This is a perfectly good answer, I do not understand the downvote.
    – copper.hat
    Aug 2 at 17:25











  • @copper.hat: In fact I did downvote, but I forgot that less 50 reputation can't comment. I tried to cancel my downvote, but it doesn't work...
    – Peter
    Aug 2 at 22:35






  • 1




    Sorry for the downvote, I forgot the fact that you can't comment. And thank you for your answer.
    – Peter
    Aug 2 at 22:36












up vote
0
down vote










up vote
0
down vote









As Mike Hawk mentioned $d_p+1$ is a linear combination of $(e_i)_1le ile p$ since it is an element of $mathbb R^p$ which has as base $(e_i)_1le ile p$






share|cite|improve this answer













As Mike Hawk mentioned $d_p+1$ is a linear combination of $(e_i)_1le ile p$ since it is an element of $mathbb R^p$ which has as base $(e_i)_1le ile p$







share|cite|improve this answer













share|cite|improve this answer



share|cite|improve this answer











answered Aug 2 at 17:06









Ahmed Lazhar

618




618











  • First, you just copy a comment (that is not very well see). Then, how do you know that $d_p+1$ is a linear combinaison of $(e_i)_1leq ileq p$ ?
    – Peter
    Aug 2 at 17:14










  • Yes I know, I did that since I am new and I cannot comment, so it was my only solution. As I told you $(e_i)_ile p$ is a base of $mathbb R^p$. And $d_p+1$ is in $mathbb R^p$
    – Ahmed Lazhar
    Aug 2 at 17:25






  • 1




    +1: This is a perfectly good answer, I do not understand the downvote.
    – copper.hat
    Aug 2 at 17:25











  • @copper.hat: In fact I did downvote, but I forgot that less 50 reputation can't comment. I tried to cancel my downvote, but it doesn't work...
    – Peter
    Aug 2 at 22:35






  • 1




    Sorry for the downvote, I forgot the fact that you can't comment. And thank you for your answer.
    – Peter
    Aug 2 at 22:36
















  • First, you just copy a comment (that is not very well see). Then, how do you know that $d_p+1$ is a linear combinaison of $(e_i)_1leq ileq p$ ?
    – Peter
    Aug 2 at 17:14










  • Yes I know, I did that since I am new and I cannot comment, so it was my only solution. As I told you $(e_i)_ile p$ is a base of $mathbb R^p$. And $d_p+1$ is in $mathbb R^p$
    – Ahmed Lazhar
    Aug 2 at 17:25






  • 1




    +1: This is a perfectly good answer, I do not understand the downvote.
    – copper.hat
    Aug 2 at 17:25











  • @copper.hat: In fact I did downvote, but I forgot that less 50 reputation can't comment. I tried to cancel my downvote, but it doesn't work...
    – Peter
    Aug 2 at 22:35






  • 1




    Sorry for the downvote, I forgot the fact that you can't comment. And thank you for your answer.
    – Peter
    Aug 2 at 22:36















First, you just copy a comment (that is not very well see). Then, how do you know that $d_p+1$ is a linear combinaison of $(e_i)_1leq ileq p$ ?
– Peter
Aug 2 at 17:14




First, you just copy a comment (that is not very well see). Then, how do you know that $d_p+1$ is a linear combinaison of $(e_i)_1leq ileq p$ ?
– Peter
Aug 2 at 17:14












Yes I know, I did that since I am new and I cannot comment, so it was my only solution. As I told you $(e_i)_ile p$ is a base of $mathbb R^p$. And $d_p+1$ is in $mathbb R^p$
– Ahmed Lazhar
Aug 2 at 17:25




Yes I know, I did that since I am new and I cannot comment, so it was my only solution. As I told you $(e_i)_ile p$ is a base of $mathbb R^p$. And $d_p+1$ is in $mathbb R^p$
– Ahmed Lazhar
Aug 2 at 17:25




1




1




+1: This is a perfectly good answer, I do not understand the downvote.
– copper.hat
Aug 2 at 17:25





+1: This is a perfectly good answer, I do not understand the downvote.
– copper.hat
Aug 2 at 17:25













@copper.hat: In fact I did downvote, but I forgot that less 50 reputation can't comment. I tried to cancel my downvote, but it doesn't work...
– Peter
Aug 2 at 22:35




@copper.hat: In fact I did downvote, but I forgot that less 50 reputation can't comment. I tried to cancel my downvote, but it doesn't work...
– Peter
Aug 2 at 22:35




1




1




Sorry for the downvote, I forgot the fact that you can't comment. And thank you for your answer.
– Peter
Aug 2 at 22:36




Sorry for the downvote, I forgot the fact that you can't comment. And thank you for your answer.
– Peter
Aug 2 at 22:36












 

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