Reverse of Cauchy-Schuarz for two vectors [closed]

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If $|x|_2leq g|y|_2$, then according to Cauchy-Schuarz, the maximum of $ langle x,y rangle$ would be $ g|y|_2^2$



$$
langle x,y rangle leq g|y|_2^2
$$
Can we prove the reverse, i.e., if $langle x,y rangle leq g|y|_2^2$, then
$$
|x|_2leq g|y|_2
$$







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closed as off-topic by Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, user223391 Jul 19 at 14:06


This question appears to be off-topic. The users who voted to close gave this specific reason:


  • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, Community
If this question can be reworded to fit the rules in the help center, please edit the question.
















    up vote
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    down vote

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    If $|x|_2leq g|y|_2$, then according to Cauchy-Schuarz, the maximum of $ langle x,y rangle$ would be $ g|y|_2^2$



    $$
    langle x,y rangle leq g|y|_2^2
    $$
    Can we prove the reverse, i.e., if $langle x,y rangle leq g|y|_2^2$, then
    $$
    |x|_2leq g|y|_2
    $$







    share|cite|improve this question











    closed as off-topic by Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, user223391 Jul 19 at 14:06


    This question appears to be off-topic. The users who voted to close gave this specific reason:


    • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, Community
    If this question can be reworded to fit the rules in the help center, please edit the question.














      up vote
      0
      down vote

      favorite









      up vote
      0
      down vote

      favorite











      If $|x|_2leq g|y|_2$, then according to Cauchy-Schuarz, the maximum of $ langle x,y rangle$ would be $ g|y|_2^2$



      $$
      langle x,y rangle leq g|y|_2^2
      $$
      Can we prove the reverse, i.e., if $langle x,y rangle leq g|y|_2^2$, then
      $$
      |x|_2leq g|y|_2
      $$







      share|cite|improve this question











      If $|x|_2leq g|y|_2$, then according to Cauchy-Schuarz, the maximum of $ langle x,y rangle$ would be $ g|y|_2^2$



      $$
      langle x,y rangle leq g|y|_2^2
      $$
      Can we prove the reverse, i.e., if $langle x,y rangle leq g|y|_2^2$, then
      $$
      |x|_2leq g|y|_2
      $$









      share|cite|improve this question










      share|cite|improve this question




      share|cite|improve this question









      asked Jul 18 at 19:27









      Saeed

      887




      887




      closed as off-topic by Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, user223391 Jul 19 at 14:06


      This question appears to be off-topic. The users who voted to close gave this specific reason:


      • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, Community
      If this question can be reworded to fit the rules in the help center, please edit the question.




      closed as off-topic by Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, user223391 Jul 19 at 14:06


      This question appears to be off-topic. The users who voted to close gave this specific reason:


      • "This question is missing context or other details: Please improve the question by providing additional context, which ideally includes your thoughts on the problem and any attempts you have made to solve it. This information helps others identify where you have difficulties and helps them write answers appropriate to your experience level." – Mostafa Ayaz, amWhy, Trần Thúc Minh Trí, Isaac Browne, Community
      If this question can be reworded to fit the rules in the help center, please edit the question.




















          1 Answer
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          No -- if $x$ is orthogonal to $y$, we can make $x$ as big as we want but still have inner product $0$






          share|cite|improve this answer




























            1 Answer
            1






            active

            oldest

            votes








            1 Answer
            1






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes








            up vote
            2
            down vote













            No -- if $x$ is orthogonal to $y$, we can make $x$ as big as we want but still have inner product $0$






            share|cite|improve this answer

























              up vote
              2
              down vote













              No -- if $x$ is orthogonal to $y$, we can make $x$ as big as we want but still have inner product $0$






              share|cite|improve this answer























                up vote
                2
                down vote










                up vote
                2
                down vote









                No -- if $x$ is orthogonal to $y$, we can make $x$ as big as we want but still have inner product $0$






                share|cite|improve this answer













                No -- if $x$ is orthogonal to $y$, we can make $x$ as big as we want but still have inner product $0$







                share|cite|improve this answer













                share|cite|improve this answer



                share|cite|improve this answer











                answered Jul 18 at 19:29









                Y. Forman

                10.8k323




                10.8k323












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