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 Dec14 awarded Yearling Dec9 awarded Caucus Sep30 awarded Explainer Jul2 awarded Curious Dec14 awarded Yearling Oct5 comment Dual space of $C_b(X)$ I would try to get this book: books.google.lt/books/about/…, Oct5 comment A problem about the quadratic form $x^TAx=0$. Usually it is assumed that matrix $A$ is symmetric when we are talking about the quadratic forms. For symmetric matrices the statement holds. Oct5 comment Solving system of linear eqaution in special cases Others pointed out that $A=ee^T-2I$, I just wanted to mention that this kind of matrix looks like Householder transformation which is well known and even was named among 10 top algorithms of 20th century. Oct5 comment A problem about the matrix equation $a^{\sf T}A_1a=0$. @Wei-ChengLiu if quadratic form $a^TBa=0$ then $B=0$. We can write any symmetric matrix as $B=P^TDP$, where $D$ is the diagonal matrix. Then $a^TBa=0$ for all $a$ is equivalent to $a^TDa=0$ for all $a$ and then it is clear that $D=0$. The comment of egreg applies to my older version of the answer, where I made some incorrect claims. Oct4 comment A problem about the matrix equation $a^{\sf T}A_1a=0$. I've removed the offending statement. We can show that $a^T(\alpha A_1+\beta A_1^T)a=0$ for all $a$ and all $\alpha$ and $\beta$, but only for $\alpha=\beta=1$ the matrix $\alpha A_1+\beta A_1^T$ is symmetric and we can invoke the quadratic form property. Oct4 revised A problem about the matrix equation $a^{\sf T}A_1a=0$. remove the incorrect statement Oct4 answered A problem about the matrix equation $a^{\sf T}A_1a=0$. May8 awarded Caucus Jan10 comment Example of atlas for sequence space @JacobSchlather, yes, open is desirable. But any example would do. Jan10 asked Example of atlas for sequence space Jan9 comment Regression with arbitrary norm Have you tried using general optimisation algorithm? Dec20 accepted Given collection of sets how to turn it to collection of disjoint sets? Dec20 comment Given collection of sets how to turn it to collection of disjoint sets? @MichaelGreinecker thanks for spotting this, I've clarified the question. I already got the answer, I needed. Dec20 revised Given collection of sets how to turn it to collection of disjoint sets? clarify the question Dec20 asked Given collection of sets how to turn it to collection of disjoint sets?