On Tuesday, 25 September 2012 14:35:17 UTC+8, Jeroen Demeyer wrote: > > On 2012-09-24 22:29, John Cremona wrote: > > It seems computationally hard to test if an element of QQbar > > is positive > What does it even mean for an element of QQbar to be positive? >
Indeed, in general this makes little sense without specifying an embedding. However, there is more to this. Indeed, say, you have f(x)=det(xI-A), and A=A^T has rational entries. E.g. that's the case when A is the adjacency matrix of an (undirected) graph. Then all the roots of a polynomial in Q[x] are real. And then the question like "are all roots of f positive?", "count positive roots", etc make perfect sense. Dima -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To post to this group, send email to sage-devel@googlegroups.com. To unsubscribe from this group, send email to sage-devel+unsubscr...@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel?hl=en.
