Christian Rau
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 Dec8 awarded Caucus Sep16 awarded Notable Question May15 awarded Yearling Apr6 awarded Popular Question Mar2 comment How many squares actually ARE in this picture? Is this a trick question with no right answer? Of course, what else. I wonder how this couldn't be a definite answer, a square is a square. May15 awarded Yearling Feb27 accepted Dihedral angles between tetrahedron faces from triangles' angles at the tip Feb27 comment Dihedral angles between tetrahedron faces from triangles' angles at the tip Since you were the first one to come up with a solution, it hurts not giving you the credit of acceptance. But I have to make a decision and achille's answer is more elaborate. But +1, of course. Feb26 comment Dihedral angles between tetrahedron faces from triangles' angles at the tip +1 Man, of course, the normal vectors with some vector arithmetics, nothing 'bout spherical trigonometry, me stupid! Feb26 comment Dihedral angles between tetrahedron faces from triangles' angles at the tip @SSumner Thanks for this link. "But I have no idea how it works" - Well, it has the formulas written on the site (though I'm also not able to directly derive this from spherical trigonometry, but at least I got a formula). Maybe somebody comes up with a nice answer showing the actual derivation of those formulas. Feb26 revised Dihedral angles between tetrahedron faces from triangles' angles at the tip added 2 characters in body Feb26 comment Dihedral angles between tetrahedron faces from triangles' angles at the tip @SSummer Really? In the end the only thing I'm uncertain of is the bottom plane of the supposed tetrahedron and this plane's location shouldn't change the dihedral angles between the other three faces. The tip's angles should completely define the tip triangles' locations relative to each other, shouldn't it? If I'm wrong on this, you could make the counter-proof an answer. Feb26 asked Dihedral angles between tetrahedron faces from triangles' angles at the tip Jun8 awarded Caucus May15 awarded Yearling Apr24 comment What is the formula of the following? I don't currently get what you mean by "the three orthogonal tangent planes". In each point of the ellipsoid you have one unique tangent plane. Or do you mean all triplets of pairwise orthogonal planes that are tangent to the ellipsoid? And of course I second the above three comments. Apr11 comment Intersection between 3D closed contour and 3D plane @msotaquira A line-plane intersection is an extremely trivial operation. Come back when your contour has thousands of points to talk about computational expense. Apr10 comment Intersection between 3D closed contour and 3D plane @msotaquira Well, then it's not a contour, is it? So you're rather asking for a good way to define a contour using those points, which is entirely dependent on the context and your needs. But using a simple piecewise linear curve, as Mark suggests in his answer (and AakashM assumed in his comment) would be a good start. Mar13 comment I have to show that the matrix $M^TM$ is SPD if and only if the columns of the matrix M are linearly independent @t.b. I have never seen it anywhere before, but Ok, according to your link it seems apprpriate. Mar13 revised I have to show that the matrix $M^TM$ is SPD if and only if the columns of the matrix M are linearly independent fixed formatting and error in last equation