diff --git a/exercises/axler-1c.md b/exercises/axler-1c.md index 8bf38de..dca8229 100644 --- a/exercises/axler-1c.md +++ b/exercises/axler-1c.md @@ -23,3 +23,70 @@ $$\{(𝑥_1, 𝑥_2, 𝑥_3)\in $\mathbf{F}^3 \mid 𝑥_1 +2 𝑥_2 +3 𝑥_3 =0 {(𝑥1,𝑥2,𝑥3) ∈ 𝐅3 ∶ 𝑥1 = 5𝑥3} +* Temp + +For each given subset, $V$, we must check that, for all +$\alpha\in\mathbf{F}$, and $v,w\in V$: +1. $\mathbf{0}\in V$; +2. $\alpha v \in V$; and +3. $v+w \in V$. + +(a) This set is a subspace. + +1. $\mathbf{0}$ is in $V$ since $\mathbf{0}=(0,0,0)$ and $0+2\times + 0+3\times =0$, as required; +2. $\alpha v$ is in $V$ since for $v=(v_1, v_2, v_3)$, we have $\alpha + v = (\alpha v_1, \alpha v_2, \alpha v_3)$ and so $(\alpha v_1) + + 2(\alpha v_3) + 3(\alpha v_3) = \alpha (v_1 + 2v_2 + 3v+3)$. The + second term is zero by supposition. +3. $v+w$ is in $v$. Write $v = (v_1, v_2, v_3)$ and $w=(w_1, w_2, + w_3)$ so that $v+w= (v_1+w_1, v_2+w_2, v_3+w_3)$. Thus $(v_1+w_1) + + 2(v_2+w_2) + 3(v_3+w_3) = (v_1+2v_2+3v_3)+(w_1+2w_2+3w_3) = 0$. + +(b) This set is not a subspace. In particular, the zero vector is not +an element since $x_1+2x_2+3x_3 \neq 4$ when $(x_1, x_2, x_3) = +(0,0,0)$ + +(c) This set is not a subspace. For example, both $(1,1,0)$ and +$(1,0,1)$ are elements (since the product of their components is zero) +but their sum, $(2,1,1)$ is not an element (since the product of its +components is not zero). + +(d) This set is a subspace. + +1. The zero vector is an element, since $0=5\times 0$. +2. Scalar multiples of elements are elements, since $x_1=5x_3$ implies + that $(\alpha x_1) = 5 (\alpha x_3)$. +3. Sums of elements are elements, since $v_1=5v_3$ and $w_1=5w_3$ + implies that $(v_1+w_1) = 5(v_3+w_3)$. + +2. (e) + +Let's do this problem for the reals first, then see whether the +corresponding claim for the complex numbers follows. + +$\mathbf{R}^\infty$ is the space of all infinite sequences of complex +numbers. For example, $(x_1, x_2, x_3, \dots)$. Some of those +sequences have a _limit_, and that limit is 0. For example, the +sequence $(1, \frac{1}{2}, \frac{1}{3}, \frac{1}{4}, \dots)$ has a limit +which is 0; the sequence $(1,1,1,\dots)$ has a limit which is 1; and +the sequence $(1,2,3,\dots)$ does not have a limit. + +In general, a sequence has a limit of 0 if "the sequence eventually +gets as close as you like to zero and stays there." More formally, the +sequence has a limit of 0 if, for any $\epsilon>0$, there is some $N$ +such that $\lvert x_n \rvert < \epsilon$ for all $n>N$. + +For example, this condition is true of the sequence $(1, \frac{1}{2}, +\frac{1}{3}, \dots, \frac{1}{n}, \dots)$. Given any $\epsilon>0$, +chose some $N > 1/\epsilon$. Then for any $n>N$, we have $\frac{1}{n} +< \frac{1}{N} < 1/\epsilon$. + +If $x_1, x_2, \dots$ is sequence with limit 0, we write +$$\lim_{n\to\infty} x_n = 0.$$ + +Are the set of these sequences a vector subspace of +$\mathbf{R}^\infty$? + +1. The zero vector is! The sequence $(0,0,0,\dotsc)$ has a limit of 0. +2. Suppose