# Math-Magic Tree filled

### More from my site

• The Trick of a Mathematical Game. The One’s Digit of the Sum of Two Numbers. Decipher the trick of the following mathematical magic.   The Rule of the Game Here is the game. Pick six natural numbers ($1, 2, 3, \dots$) and place them in the yellow discs of the picture below. For example, let's say I have chosen the numbers $7, 5, 3, 2, […] • Every Group of Order 12 Has a Normal Subgroup of Order 3 or 4 Let$G$be a group of order$12$. Prove that$G$has a normal subgroup of order$3$or$4$. Hint. Use Sylow's theorem. (See Sylow’s Theorem (Summary) for a review of Sylow's theorem.) Recall that if there is a unique Sylow$p$-subgroup in a group$GH$, then it is […] • Find a Matrix so that a Given Subset is the Null Space of the Matrix, hence it’s a Subspace Let$W$be the subset of$\R^3$defined by $W=\left \{ \mathbf{x}=\begin{bmatrix} x_1 \\ x_2 \\ x_3 \end{bmatrix}\in \R^3 \quad \middle| \quad 5x_1-2x_2+x_3=0 \right \}.$ Exhibit a$1\times 3$matrix$A$such that$W=\calN(A)$, the null space of$A$. […] • Complex Conjugates of Eigenvalues of a Real Matrix are Eigenvalues Let$A$be an$n\times n$real matrix. Prove that if$\lambda$is an eigenvalue of$A$, then its complex conjugate$\bar{\lambda}$is also an eigenvalue of$A$. We give two proofs. Proof 1. Let$\mathbf{x}$be an eigenvector corresponding to the […] • The Sum of Subspaces is a Subspace of a Vector Space Let$V$be a vector space over a field$K$. If$W_1$and$W_2$are subspaces of$V$, then prove that the subset $W_1+W_2:=\{\mathbf{x}+\mathbf{y} \mid \mathbf{x}\in W_1, \mathbf{y}\in W_2\}$ is a subspace of the vector space$V$. Proof. We prove the […] • Dot Product, Lengths, and Distances of Complex Vectors For this problem, use the complex vectors $\mathbf{w}_1 = \begin{bmatrix} 1 + i \\ 1 - i \\ 0 \end{bmatrix} , \, \mathbf{w}_2 = \begin{bmatrix} -i \\ 0 \\ 2 - i \end{bmatrix} , \, \mathbf{w}_3 = \begin{bmatrix} 2+i \\ 1 - 3i \\ 2i \end{bmatrix} .$ Suppose$\mathbf{w}_4$is […] • A Simple Abelian Group if and only if the Order is a Prime Number Let$G$be a group. (Do not assume that$G$is a finite group.) Prove that$G$is a simple abelian group if and only if the order of$G$is a prime number. Definition. A group$G$is called simple if$G$is a nontrivial group and the only normal subgroups of$G$is […] • Is the Set of Nilpotent Element an Ideal? Is it true that a set of nilpotent elements in a ring$R$is an ideal of$R$? If so, prove it. Otherwise give a counterexample. Proof. We give a counterexample. Let$R$be the noncommutative ring of$2\times 2\$ matrices with real […]