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1. • CommentRowNumber1.
   • CommentAuthorUrs
   • CommentTimeMar 30th 2019
   • PermaLink
   Author: Urs
   Format: MarkdownItexadded pointer to * [[Hirosi Ooguri]], Zheng Yin, _TASI Lectures on Perturbative String Theories_ ([arXiv:hep-th/9612254](https://arxiv.org/abs/hep-th/9612254)) But I was really looking for a solid reference for the basic statement that the maximal compact subgroup of $O(n,n)$ is $O(n) \times O(n)$. <a href="https://ncatlab.org/nlab/revision/diff/Narain+group/2">diff</a>, <a href="https://ncatlab.org/nlab/revision/Narain+group/2">v2</a>, <a href="https://ncatlab.org/nlab/show/Narain+group">current</a>

   added pointer to

   • Hirosi Ooguri, Zheng Yin, TASI Lectures on Perturbative String Theories (arXiv:hep-th/9612254)

   But I was really looking for a solid reference for the basic statement that the maximal compact subgroup of $O(n,n)$ is $O(n) \times O(n)$.

   diff, v2, current

2. • CommentRowNumber2.
   • CommentAuthorTodd_Trimble
   • CommentTimeMar 30th 2019
   • PermaLink
   Author: Todd_Trimble
   Format: MarkdownItexThere's a section on maximal compact subgroups of the indefinite orthogonal group [here](http://virtualmath1.stanford.edu/~conrad/210CPage/handouts/maxcompact.pdf).

   There’s a section on maximal compact subgroups of the indefinite orthogonal group here.