This set is due February 6 at the beginning of lecture. Consult the syllabus for details on the homework policy.

1. a. Complete the proof by induction that if are integers and , then for all integers .

b. This result allows us to give a nice proof of the following fact: Let be a natural number and let be a positive integer. If the -th root of , , is rational, then it is in fact an integer. (The book gives a proof of a weaker fact.) Prove this result as follows: First verify that if and , then . Show that any fraction with integers, can be reduced so . Assume that is rational, say . Then also . Express this last fraction as a rational number in terms of . Use that for all and the general remarks mentioned above, to show that is in fact an integer.

2. Show by induction that for all integers there is a polynomial with rational coefficients, of degree and leading coefficient , such that for all integers , we have There are many ways to prove this result. Here is one possible suggestion: Consider .

3. Euclidean algorithm. We can compute the gcd of two integers not both zero, as follows; this method comes from Euclid and is probably the earliest recorded algorithm. Fist, we may assume that are positive, since , and also we may assume that , so . Now define a sequence of natural numbers as follows:

, .

Given , if , then .

Otherwise, , and we can use the division algorithm to find unique integers with such that . Set .

Let be the set of those that are strictly positive. This set has a least element, say . By the way the algorithm is designed, this means that .

Show that , and that we can find from the algorithm, integers such that .

(If the description above confuses you, it may be useful to see the example in the book.)

4. Assume that the application of the algorithm, starting with positive integers , takes steps. [For example, if and , the algorithm gives:

Step 1: , so .

Step 2: , so .

Step 3: , so , and . Here, ]

Show that , where the numbers are the Fibonacci numbers, see Exercises 15-22 in Chapter 1 of the book.

5. Extra credit problem. With as in the previous exercise, let be the number of digits of (written in base 10). Show that

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[…] Homework 1, due February 6, at the beginning of lecture. Possibly related posts: (automatically generated)What Lil’ Ones Are Reading: Two Christmas Stories […]

Perhaps this is a stupid questions, but what does the notation (a,b) = 1 mean in the first problem? Is it a function that I have missed somewhere? Is it equivalent to a = b = 1, so that we are to prove that a = a^n = 1 and b = b = 1 for all integers n >= 1? I’m a little lost…

Hi. The notations and mean the same thing: The greatest common divisor of and .
[I agree there is a little ambiguity in using this way, but it is standard in number theory.]

No, this is not consistent. Todorčević has shown in ZF that, in fact, there is no function $F\!:\mathcal W(S)\to S$ with the property you require. Here, $\mathcal W(S)$ is the collection of subsets of $S$ that are well-orderable. This is corollary 6 in MR0793235 (87d:03126). Todorčević, Stevo. Partition relations for partially ordered sets. Acta Math. 155 (1 […]

As suggested by Gerald, the notion was first introduced for groups. Given a directed system of groups, their direct limit was defined as a quotient of their direct product (which was referred to as their "weak product"). The general notion is a clear generalization, although the original reference only deals with groups. As mentioned by Cameron Zwa […]

A database of number fields, by Jürgen Klüners and Gunter Malle. (Note this is not the same as the one mentioned in this answer.) The site also provides links to similar databases.

As the other answer indicates, the yes answer to your question is known as the De Bruijn-Erdős theorem. This holds regardless of the size of the graph. The De Bruijn–Erdős theorem is a particular instance of what in combinatorics we call a compactness argument or Rado's selection principle, and its truth can be seen as a consequence of the topological c […]

Every $P_c$ has the size of the reals. For instance, suppose $\sum_n a_n=c$ and start by writing $\mathbb N=A\cup B$ where $\sum_{n\in A}a_n$ converges absolutely (to $a$, say). This is possible because $a_n\to 0$: Let $m_0

Sure. A large class of examples comes from the partition calculus. A simple result of the kind I have in mind is the following: Any infinite graph contains either a copy of the complete graph on countably many vertices or of the independent graph on countably many vertices. However, if we want to find an uncountable complete or independent graph, it is not e […]

I think that, from a modern point of view, there is a misunderstanding in the position that you suggest in your question. Really, "set theory" should be understood as an umbrella term that covers a whole hierarchy of ZFC-related theories. Perhaps one of the most significant advances in foundations is the identification of the consistency strength h […]

I'll only discuss the first question. As pointed out by Asaf, the argument is not correct, but something interesting can be said anyway. There are a couple of issues. A key problem is with the idea of an "explicitly constructed" set. Indeed, for instance, there are explicitly constructed sets of reals that are uncountable and of size continuum […]

The question seems to be: Assume that there is a Vitali set $V$. Is there an explicit bijection between $V$ and $\mathbb R$? The answer is yes, by an application of the Cantor-Schröder-Bernstein theorem: there is an explicit injection from $\mathbb R$ into $\mathbb R/\mathbb Q$ (provably in ZF, this requires some thought, or see the answers to this question) […]

If a set $X$ is well-founded (essentially, if it contains no infinite $\in$-descending chains), then indeed $\emptyset$ belongs to its transitive closure, that is, either $X=\emptyset$ or $\emptyset\in\bigcup X$ or $\emptyset\in\bigcup\bigcup X$ or... However, this does not mean that there is some $n$ such that the result of iterating the union operation $n$ […]

[…] Homework 1, due February 6, at the beginning of lecture. Possibly related posts: (automatically generated)What Lil’ Ones Are Reading: Two Christmas Stories […]

Perhaps this is a stupid questions, but what does the notation (a,b) = 1 mean in the first problem? Is it a function that I have missed somewhere? Is it equivalent to a = b = 1, so that we are to prove that a = a^n = 1 and b = b = 1 for all integers n >= 1? I’m a little lost…

Hi. The notations and mean the same thing: The greatest common divisor of and .

[I agree there is a little ambiguity in using this way, but it is standard in number theory.]

[…] Homework 1, due February 6, at the beginning of lecture. [This homework was not graded.] […]