Christian Rau
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 Feb 9 suggested approved edit on transformation of 3D coordinate system Feb 9 revised transformation of 3D coordinate system fixed formatting Feb 9 suggested approved edit on transformation of 3D coordinate system Feb 9 comment Let $\rho(P)$ be the spectral radius of $P$. Show $\rho( \dfrac{P}{ \rho(P) + \epsilon } ) < 1 \text{ for all } \epsilon >0.$ Can it be you're missing a $c$ in there (first equation)? Otherwise I cannot believe $\lambda$ being an eigenvalue implies $c\lambda$ being an eigenvalue of the same matrix, too. With $\sigma$ you mean the spectrum, right? Feb 8 answered Relationship between nullspace and row/column space Feb 7 comment matrix equation $(A-B)CA=B$ It may be simple, but why does $Q(CA+I)=A$ make $Q$ invertible? Feb 3 comment 3D Rotation Matrix Uniqueness @RahulNarain Hah, indeed! Feb 3 revised linear interpolation in 3 dimensions fixed formatting Feb 3 suggested approved edit on linear interpolation in 3 dimensions Feb 3 answered (Graphics Gems IV, Shoemake) From matrix to euler angles explanation Feb 3 comment 3D Rotation Matrix Uniqueness @Korchkidu Ok, updated my answer. Feb 3 revised 3D Rotation Matrix Uniqueness added 643 characters in body Feb 3 comment 3D Rotation Matrix Uniqueness @Korchkidu Well, in fact it's more intuition, since it has to hold for all $\mathbf{v}\in\mathbb{R}^n$, for each and every matrix $M\neq O$ you can always find a $\mathbf{v}$ for which $M\mathbf{v}\neq\mathbf{0}$, but I'm sure there is also some theorem or prove out there for this. Feb 3 answered Is there a classic Matrix Algebra reference? Feb 3 answered 3D Rotation Matrix Uniqueness Feb 3 comment 3D Rotation Matrix Uniqueness Actually the rotation matrix $R'$ for the negated axis and angle (that you don't consider the same as $R$) is in fact the exact same matrix as the original $R$. So the matrix representation is even more unique than the axis-angle or quaternion representation. Feb 3 answered How to get a projected 3d line segment, lie on another 3d line parallel to that line segment. Feb 3 comment What is the conjugate of $\frac{1}{2}+ \frac{3}{2}i$? @David There is nowhere to arrive from, conjugation is just defined this way. period. Feb 2 comment Multiplying double-centered matrix to a unit vector @user506901 But like said, such a translation is not achievable using a matrix-multiplication (and therefore neither with a double-centred matrix). Centering a vector is no translation, since the summands required to center the vector are no constants but depend on the vector itself. Centering a different vector (using the same matrix) would result in different summands, thus the application of the matrix doesn't represent a translation. Feb 2 comment Multiplying double-centered matrix to a unit vector @user506901 Or do you mean adding a different constant to each entry? This sounds more like a translation, but again, this cannot be done with a vector (maybe you mean a vector representing the location vector of a point?). And this cannot be achieved by a matrix multiply, because all summands added to the vector entries depend on the vector itself and cannot be constants. Maybe you are mixing things up here and think of the $\mathbb{R}^{n+1}$ projective space usually used in computer graphics and the like. In this space translation can indeed be realized by a matrix-multiply?