How about $M_n(\mathbb{R})^+ http://latex.mathoverflow.net/png?%24M%5Fn%28%5Cmathbb%7BR%7D%29%5E%2B? I have seen S^+ http://latex.mathoverflow.net/png?S%5E%2B or S_+ http://latex.mathoverflow.net/png?S%5F%2B used to denote the set of positive linear transformations in a set S http://latex.mathoverflow.net/png?S of linear transformations on an inner product space, but this was in the context of operator algebras.
Jonas Meyer
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