Luboš Motl
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 Jul 9 answered Compositions of prime numbers Jun 27 comment Validity of $\sum_{i=1}^n(a_i^2+b_i^2+c_i^2+d_i^2)\lambda_i\geq\lambda_1+\lambda_2+\lambda_3+\lambda_4$? Oh, I see, you're right. Jun 27 comment Puzzle: $(\Box @)+(\Box @) = (\Box\bigstar\Box$) Thanks for your care and implicit compliments, @JTL and @Jyrki, but I don't think it's a big issue. Let's be generous, people have the right to vote the way they want. Jun 27 awarded Nice Answer Jun 26 revised Validity of $\sum_{i=1}^n(a_i^2+b_i^2+c_i^2+d_i^2)\lambda_i\geq\lambda_1+\lambda_2+\lambda_3+\lambda_4$? added 99 characters in body; added 570 characters in body; added 2 characters in body Jun 26 revised Puzzle: $(\Box @)+(\Box @) = (\Box\bigstar\Box$) added 123 characters in body Jun 26 revised Validity of $\sum_{i=1}^n(a_i^2+b_i^2+c_i^2+d_i^2)\lambda_i\geq\lambda_1+\lambda_2+\lambda_3+\lambda_4$? added 454 characters in body; added 39 characters in body; added 55 characters in body; added 47 characters in body Jun 26 comment Validity of $\sum_{i=1}^n(a_i^2+b_i^2+c_i^2+d_i^2)\lambda_i\geq\lambda_1+\lambda_2+\lambda_3+\lambda_4$? Interesting, I jumped on the same intuition as you did - it couldn't be right, I thought, because the RHS didn't have the minimal $\lambda$ anymore. Of course, the catch is that the "generalization" has almost nothing to do with the original inequality. Jun 26 answered Validity of $\sum_{i=1}^n(a_i^2+b_i^2+c_i^2+d_i^2)\lambda_i\geq\lambda_1+\lambda_2+\lambda_3+\lambda_4$? Jun 26 comment How to take the inverse of this signal? You were faster, I erased my answer. ;-) Jun 26 answered Puzzle: $(\Box @)+(\Box @) = (\Box\bigstar\Box$) Jun 26 answered Eigenvectors of a normal matrix Jun 26 answered How common is the use of the term “primitive” to mean “antiderivative”? Jun 26 comment Plane intersecting line segment The objects you called $dist$ are not distances in the exact sense. They're inner products which can be both positive and negative. There's no reason to divide the debate to the positive and negative case because all the geometrically natural formulae here are linear, rational, or otherwise analytic and they work uniformly both for positive and negative values of the inner products. Jun 26 comment Plane intersecting line segment Dear @PUK, if a line belongs to a plane, then it surely is parallel with it. At any rate, when it is so, the intersection - as in set theory - of the plane and the line is the whole line (the very same one). I don't understand in what sense it would make sense to pick two particular points on the line as "better intersections" than all the other points. Also, I am confused by your separate discussion of negative and positive values of $dist.dist$. A fun about maths and algebra is that one may calculate with negative numbers just like with the positive ones w/o splitting derivations. Jun 25 revised Plane intersecting line segment added 66 characters in body Jun 25 revised Plane intersecting line segment added 32 characters in body Jun 25 answered Plane intersecting line segment Jun 18 answered How to partial differential of a trace of matrix form? Jun 16 answered “Closest pair of points” algorithm