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Certain Number-Theoretic Episodes In Algebra

R Sivaramakrishnan

Chapter 8

Ordered fields, fields with valuation and other algebraic structures - all with Video Answers

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Chapter Questions

Problem 1

Mark the following statements true $(T)$ or false $(F)$ justifying your answer briefly.
a) Let $D$ be an ordered integral domain with positive subset $P$. In $D[x]$, we write

$$
\left(a_0+a_1 x+\cdots+a_n x^n\right) \in P^*, a_n \neq 0
$$

if, and only if, $a_n \in P$ in $D$. Then, $D[x]$ with positive set $P^*$ is an ordered integral domain.
b) $\mathbb{Z}[\sqrt{3}]=\{a+b \sqrt{3}: a, b \in \mathbb{Z}\}$ can be made an ordered integral domain, by defining the set $P$ of positive elements.
c) Let $n=p_1^{a_1}, p_2^{a_2}, \ldots, p_k^{a_k}\left(p_1<p_2 \cdots<p_k\right)\left(p_i\right.$, primes, $\left.i=1,2, \ldots, k\right)$ $D(n)$, the set of divisors of $n$ partially ordered by divisibility forms a distributive lattice $L_1 \times L_2 \times \cdots L_k$ where $L_i$ is a chain of length $a_i(i=1,2, \ldots, k)$.
d) There exists a modular lattice of 7 elements in which the complemented elements do not form a sublattice.
e) Any Boolean algebra generated by $n$ elements has more than $2^{2^n}$ elements.
f) The modular lattice of all subspaces of a finite dimensional vector space $V$ is also a complemented lattice.

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Problem 2

An ordered integral domain $D$ is said to be complete, if every non-empty set of positive elements of $D$ has a g.l.b in D. Prove that the set $\mathbb{R}$ of real numbers is a complete ordered integral domain.

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Problem 3

[S. Lang]
(a) Starting from an absolute value map $m: \mathbb{Q} \rightarrow \tilde{\mathbb{R}}$, describe the archimedean and non-archimedean prime divisors of $Q$. (the field of rational numbers).
(b) Let $\mathbb{Q}_p$ denote the field of p-adic numbers where $p$ is a rational prime. Show that $\mathbb{Q}_p$ contains infinitely many fields of the type $\mathbb{Q}(\sqrt{-m})$, where $m$ is a positive integer.

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Problem 4

Prove that a lattice $(L, \vee, \wedge)$ is distributive, if the equations

$$
x \vee y=x \vee z \text { and } x \wedge y=x \wedge z \Longrightarrow y=z
$$

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Problem 5

Suppose that we consider the lattice of normal subgroups of a group G. Show that it need not be a modular lattice. Check this for the lattice of normal subgroups of the Klein 4-group

$$
V=\{e, a, b, c\} \text { with } a b=b c=c a, \quad a^2=b^2=c^2=e .
$$

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Problem 6

Let $F=\mathbb{Q}[x] /\left(x^2+r\right), r>0$. Show that $F$ can be ordered in two different ways. (One has to consider the monomorphisms of $F$ into $\mathbb{C}$ ).

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Problem 7

Let $(B,+, \cdot)$ be a ring containing 0 and 1 and for $a \in B$, the complement of a denoted by $\bar{a}=a+1$ and

$$
a+b=a \cdot \bar{b}+\bar{a} \cdot b, \quad a \cdot 1=a, \quad a \cdot \bar{a}=a+a=0
$$

Check whether B is a Boolean algebra.

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Problem 8

Let $(B,+, \cdot)$ be a Boolean algebra. For $a, b \in B$, show that $a=a \cdot b$ if, and only if, $a \cdot \bar{b}=0$ where $\bar{b}$ is the complement of $b$ in $B$.

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03:57

Problem 9

Suppose that $L_1$ and $L_2$ are lattices which are isomorphic as posets. Show that $L_1$ and $L_2$ are isomorphic as lattices.

Prathan Jarupoonphol
Prathan Jarupoonphol
Numerade Educator

Problem 10

Give an example of a lattice $(L, \vee, \wedge)$ with a subset $L_1$ such that $\left(L_1, \vee, \wedge\right)$ is a lattice, but not a sublattice of $L$.

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Problem 11

Prove that the positive divisors of a positive integer $n$ form a complemented lattice if, and only if, $n$ is square-free.

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