A product sum inequality question - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-24T04:01:05Z http://mathoverflow.net/feeds/question/83227 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/83227/a-product-sum-inequality-question A product sum inequality question unknown (yahoo) 2011-12-12T07:33:10Z 2011-12-20T06:53:12Z <p>For any $x_{1},x_{2},\cdots x_{6}$ with $\sum_{i=1}^{6}x_{i}^{2}=1$ and $y_{1},y_{2},\cdots y_{6}$ in $\mathbb{R}$ with $\sum_{i=1}^{6}y_{i}^{2}=1$, do there always exist $z_{1},z_{2},\cdots z_{6}$ in $\mathbb{R}$ with $\sum_{i=1}^{6}z_{i}^{2}=6$ such that $\left|z_{1}z_{2}z_{3}z_{4}\sum_{i=1}^{6}x_{i}z_{i}\sum_{j=1}^{6}y_{j}z_{j}\right|\ge1$?</p> <p>For special cases such as $x_iy_i\ge0$, one can see it holds. But for the general case, I am stuck. Could anyone help on this question?</p> <p>Any helpful answer would be greatly appreciated!</p>