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In matrix notation what does it mean X 'X I see it in this academic paper, I can't find it in matrix operation and can't search with google.

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2400219

page 7 , paragraph 3.1

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    $\begingroup$ I'd guess that $X'$ means transpose, and $X'X$ is the product of two matrices. $\endgroup$ – user147263 Feb 13 '15 at 3:11
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    $\begingroup$ Yeah, it's a shame. It's not like LaTeX can't just as easily produce $X^T$ or anything. $\endgroup$ – Ken Feb 13 '15 at 6:16
  • $\begingroup$ en.wikipedia.org/wiki/Transpose $\endgroup$ – lumbric May 17 '18 at 17:23
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$X'$ is the transpose matrix and $X'X$ is the product the same matrix. exemple $$X=\left(\matrix{1 & 2&4\\1&3&5}\right),$$ then, $$X'X=\left(\matrix{1 & 1\\2&3\\4&5}\right)\left(\matrix{1 & 2&4\\1&3&5}\right).$$

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