Beni Bogosel
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 Nov 18 comment If n $\geq2$, does G necessarily have an element of order $p^2$? Justify your answer. @John Yes, but the counterexample still works :) Nov 18 comment If n $\geq2$, does G necessarily have an element of order $p^2$? Justify your answer. What is $G$, what is $p$? Nov 10 comment Conditional probability elementary problem - check if I am right But, indeed, the probability that a girl opens the door, is not equal to the probability that there exists a girl in the family. Nov 10 comment Conditional probability elementary problem - check if I am right If I use conditional probabilities for $(B)$ I get $P(\text{there are two boys} | \text{one child is a girl}) = P(\text{two boys and a girl})/P(\text{one child is a girl}) = \frac{3/8}{7/8}=\frac{3}{7}$. Nov 10 comment Conditional probability elementary problem - check if I am right :)) Yes. Sorry about that. Sep 22 comment Is this bootstrap argument correct? You said "the trace is preserved under weak convergence". Can you please give a reference where this result can be found? Sep 15 comment basic question involving topology and the Hausdorff distance If you can read a bit in French, I recommend springer.com/mathematics/book/978-3-540-26211-4 In chapter 2 there is a detailed discussion of different types of convergence of sets. There is an exercice which resembles your question. If you prove that $\Omega_n$ converges to $\Omega$ in the Hausdorff distance, then the convexity should imply the convergence of the boundaries. Aug 15 comment Families of Square Roots of Identity Matrices You can tackle the general case using reduction (diagonalization). Aug 9 comment Distance between triangle's centroid and incenter, given coordinates of vertices The sides of the triangle can be immediately calculated. After that, the coordinates of $G$ are just the averages of the coordinates of the vertices. For the incenter the formula is simple: you can find it here mathworld.wolfram.com/Incenter.html In my opinion there's no simpler way. Jul 17 comment How prove $A^2=0$,if $AB-BA=A$ @Kelenner: I'm sorry. I didn't make all the computations. Jul 17 comment How to find the determinant of this matrix @user143993: This last equivalence you wrote is the definition of dependent vectors. Jul 17 comment How to find the determinant of this matrix @user143993: The statement you wrote is not right. The determinant is zero if and only if the columns (or the lines) are related. You can prove it like this: if the determinant is not zero, then any relation between the columns implies that the coefficients are zero (because the system has the unique solution of zeros). Therefore the columns are not related. Conversely, if the determinant is zero, then the system doesn't have a unique solution, i.e. has a non zero solution, i.e. you can find a relation between the column vectors. Jul 17 comment How to find the determinant of this matrix No, there is no $f$ here. The equation still defines a conic. Jul 17 comment How prove $A^2=0$,if $AB-BA=A$ @Kelenner: I think it is $2^kA^k=A^kB-BA^k$. To prove that $A$ is nilpotent it is enough to prove that $tr(A^k)=0$ for each $k$, which you did. After that you can deduce that all eigenvalues are zero using a Vandermonde system. Jul 17 comment Disconnecting using totally disconnected sets This was already asked here: math.stackexchange.com/questions/31667/… and here: mathoverflow.net/questions/55718/… Jul 13 comment Quaternion Group as Permutation Group @angryavian: The phrase: "I couldn't figure out how to do it" says the opposite. The wolfram citation may be wrong, since it contradicts Cayley's theorem. Jul 13 comment Quaternion Group as Permutation Group See what the application $f(x)=ax$ does with the elements of the group. That is the permutation corresponding to $a$. Do this for every $a$ to find the permutation representartion. Jul 9 comment Clarification: Suppose that $(v_1, \ldots ,v_n)$ is a basis of $V$ and $(w_1, \ldots ,w_n)$ is the basis of $W$ … Yes, I get what you mean, but if you got this result from a book, you should search for the DEFINITION of M. $M$ is not arbitrary. The idea is that to every two bases you associate a matrix, and it is natural to think of the matrix associated to a linear application $T$, especially if in the proof you can see $M(T)$. Look for the definition of that matrix, which must be in there somewhere. $M(T)$ cannot be just an arbitrary matrix, because then the application you consider does not need to be linear and invertible. Jul 9 comment Clarification: Suppose that $(v_1, \ldots ,v_n)$ is a basis of $V$ and $(w_1, \ldots ,w_n)$ is the basis of $W$ … You could look at the definitions before this proposition and figure out what $M$ is... Jul 8 comment Evaluate $\int_{0}^{\frac{\pi}2}\frac1{(1+x^2)(1+\tan x)}\:\mathrm dx$ What is the source of this problem?