Thursday, 24 July 2014

recreational mathematics - Help understanding proof of generalization of Cauchy-Schwarz Inequality



I'm having trouble with an exercise in the Cauchy Schwarz Master Class by Steele. Exercise 1.3b asks to prove or disprove this generalization of the Cauchy-Schwarz inequality:




enter image description here



The following is the solution at the end of the book:



enter image description here



After struggling to understand the solution for a few hours, I still cannot see why the substitution c2k/(c21++c2n) would bring the target inequality to the solvable inequality. Neither do I understand what the n2<n3 bound has to do with anything or how it allows us to take a "cheap shot".



Thanks!




Edit: I'm also wondering, is there a name for this generalization of Cauchy-Schwarz? Any known results in this direction?


Answer



I have reason to believe the text has a typo; maybe someone can correct me on this point. Because, to my mind, the definition of the ˆci's would apply to the sum |akbkc2k|. I suspect it should read



ˆci=cic21+c22++c2n.






If my hunch is correct, we would argue as follows (using the ˆci's defined right above):




|nk=1akbkˆck|nk=1|akbkˆck|nk=1|akbk|=|nk=1|ak||bk||(nk=1|ak|2)1/2(nk=1|bk|2)1/2



Above:





  • : Follows from triangle inequality

  • : Follows from |ˆck|1, k=1,,n.

  • =: Follows because x=|x| for x0.

  • : Cauchy-Schwarz applied to |ak|,|bk|.



Now take the far left and far right side of this, square, and multiply by c21+c22+c2n (apply to ˆci).


No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find limh0sin(ha)h without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...