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Math 402 - 01 Previous Homework (Spring 2019)

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Problem Set 03 (complete) Due: 02/18/2019 Board presentation: 02/20/2019

1. Let $V$ be a vector space and $B\subseteq V$. Show that the following are equivalent
1. $B$ is a basis for $V$,
2. $B$ is maximal linearly independent set,
3. $B$ is minimal spanning set.
2. Let $V$ be a vector space and $W$ a subspace of $V$.
1. Prove that $\dim(W) \leq \dim(V)$.
2. Prove that if $V$ is finite dimensional and $\dim(W)=\dim(V)$ then $W=V$
3. Show, with a counterexample, that the finite dimensional hypothesis is necessary in part b.
3. In regards to the Universal Mapping Property for vector spaces discussed in class today:
1. Complete the proof of it.
2. Prove that the set $\{\alpha(v)\mid v\in B\}$ is linearly independent in $W$ iff $\widehat{\alpha}$ is injective.
3. Prove that the set $\{\alpha(v)\mid v\in B\}$ is a spanning set for $W$ iff $\widehat{\alpha}$ is surjective.
4. Let $V$ be a vector space over $F$, and $W$ a subspace of $V$. Let $B_1$ be a basis for $W$ and $B$ a basis for $V$ such that $B_1\subseteq B$. Prove that the set $\{v+W\mid v\in B-B_1\}$ is a basis for the quotient space $V/W$.

Problem Set 02 (complete) Due: 02/11/2019 Board presentation: 02/18/2019

1. Let $D$ be a UFD. $a,b,c\in D$, and $f(x)\in D[x]$. $a,b$ are said to be ”relatively prime” if $\gcd(a,b)$ is a unit.
1. Prove that if $a,b$ are relatively prime and $a|bc$ then $a|c$.
2. Prove that if $\frac{a}{b}$ is a root of $f(x)$, and $a,b$ are relatively prime, then $a$ divides the constant term of $f(x)$ and $b$ divides the leading term of $f(x)$.
2. Let $D$ be an ED, $a,b\in D$, with $b\neq 0$. Consider the sequence $r_0,r_1,r_2,\dots,r_n$ defined recursively as follows: $r_0=a,r_1=b$, and using the propery of an Euclidean Domain, until obtaining a residue $0$, $\begin{array}{rclll} r_0 &=&q_1 r_1 + r_2 &\text{ and} &\delta(r_2) < \delta(r_1), \\ r_1 &=&q_2 r_2 + r_3 &\text{ and} &\delta(r_3) < \delta(r_2), \\ &\vdots \\ r_{n-3} &=&q_{n-2} r_{n-2} + r_{n-1} &\text{ and} &\delta(r_{n-1}) < \delta(r_{n-2}), \\ r_{n-2} &=&q_{n-1} r_{n-1} + r_n &\text{ and} &r_n=0. \\ \end{array}$ Why does the sequence $r_1,r_2,\dots,r_n$ have to eventually attain the value $r_n=0$? Prove that the last non-zero entry in the residues list, i.e. $r_{n-1}\sim\gcd(a,b)$.
3. Let $D$ be a PID, $a,b\in D$. Let $d$ be a generator of the ideal $\pbr{a}+\pbr{b}$. Show that $d\sim\gcd(a,b)$.
4. Let $D$ be an ID, $a,b\in D$. Prove that if $a$ and $b$ have a least common multiple $l\in D$, then $\frac{ab}{l}$ is a greatest common divisor of $a$ and $b$ in $D$.
5. (Optional) Let $\gamma=\ds\frac{1+\sqrt{-19}}{2}$ and consider the subring of $\C$ given by: $R = \{a + b\gamma\mid a,b\in\Z\}$ Prove that $R$ is a PID but not an ED. A detailed proof can be found in Mathematics Magazine, Vol. 46, No. 1 (1973), pp 34-38. If you choose to work on this problem, do not consult this reference, or any other reference. Hand-in only your own work, even it it is only parts of the solution.

Problem Set 01 (complete) Due: 02/01/2019 Board presentation: 02/08/2019

1. Let $D$ be an integral domain. Consider the following two properties that $D$ and a function $\delta:D-\{0\}\to\N_0$ may have:
1. For any $a,d\in D$ with $d\neq 0$, there are $q,r\in D$ such that
$a=qd+r$ and ( $r=0$ or $\delta(r) < \delta(d))$
2. For any $a,b\in D-\{0\}$, $\delta(a)\leq\delta(ab)$.
Prove that if there is a function $\delta$ satisfying the first condition, then there is a function $\gamma$ satisfying both of them. Hint: consider $\gamma$ defined by: $\gamma(a):= \min_{x\in D-\{0\}}\delta(ax)$
2. Chapter 18, problem 22.
3. Chapter 16, problem 24. Can you weaken the assumption “infinitely many”?
4. Show that an integral domain $D$ satisfies the ascending chain condition ACC iff every ideal of $D$ is finitely generated. (Hint: one direction is similar to the proof that every PID satisfies the ACC).