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Aug 1, 2020 at 3:24 history became hot network question
Jul 31, 2020 at 20:24 history edited YCor CC BY-SA 4.0
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Jul 31, 2020 at 20:09 vote accept Malkoun
Jul 31, 2020 at 20:08 answer added Robert Bryant timeline score: 16
Jul 31, 2020 at 20:02 comment added Malkoun @RobertBryant, could you please write it as an answer? The answer turned out to be simple (and I should have thought about it), but it is guiding me in the right direction (for the problem I am interested in, which inspired this post). Is the post too trivial? Should I delete it?
Jul 31, 2020 at 19:55 comment added Malkoun @RobertBryant, ah yes true! The famous sum of squares trick, when working over $\mathbb{R}$. Thank you. How can one obtain all such polynomials? Can one use some form of the positivstellensatz perhaps?
Jul 31, 2020 at 19:52 comment added Robert Bryant Actually, there's an octic polynomial: $$Q(v_1,v_2,\ldots,v_n) = \sum_{1\le i<j\le n} ((v_i,v_i)(v_j,v_j)-(v_i,v_j)^2)^2.$$
Jul 31, 2020 at 19:49 comment added Malkoun @KevinCasto, yes but, the determinant of the Gramian vanishes iff the vectors are linearly dependent. What I would like is though, a polynomial which vanishes iff the vectors lie in the same $1$-dimensional subspace.
Jul 31, 2020 at 19:35 comment added Kevin Casto Indeed the Gramian is positive semi-definite, so its determinant is always nonnegative, and is positive just when the vectors are linearly independent. See en.wikipedia.org/wiki/Gramian_matrix#Gram_determinant
Jul 31, 2020 at 19:31 history edited Malkoun CC BY-SA 4.0
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Jul 31, 2020 at 19:22 history asked Malkoun CC BY-SA 4.0