Math 502 course site

- No Homework this week

- Kunen, exercise II.15.5. Suppose that $T$ is a theory in which every sentence consists of universal quantifiers followed by a quantifier-free formula. Show that if $\mathcal A\models T$ and $\mathcal B\subset\mathcal A$, then $\mathcal B\models T$.
- Show that $\mathbb R$ and $\mathbb R\setminus{0}$ are not isomorphic as linear orders.
- Kunen, exercise II.17.16. Show that HF is a model of the sentence which states that every element is in bijection with a natural number.
- … Choose your own!

- Let $T$ be the theory of arithmetic, that is, the sentences true in the structure $(\mathbb N;+,\cdot,0,1,<)$. Show there is a model of $T$ with an initial segment isomorphic to $\mathbb N$, and additional “infinite” numbers greater than $\mathbb N$.
- Show that the class of all finite graphs is not first-order axiomatizable (that is, there is no theory $\Sigma$ such that the models of $\Sigma$ are exactly the finite graphs).
- Let $T$ be a complete theory with an infinite model. Can $T$ have any finite models?

- Let $\mathcal A$ be a nonstandard model of the theory of arithmetic. By the above exercise we know that $\mathcal A$ has an initial segment isomorphic to $\mathbb N$. Show that this initial segment has no supremum.
- Let $\mathcal A$ be a nonstandard model of the theory of arithmetic. Show that $\mathcal A$ contains an element which is divisible by every (standard) prime number $p$.
- Let $\mathcal A$ be a nonstandard model of the theory of arithmetic. Show that $\mathcal A$ contains an “infinite” prime number.
- Show that the class of all infinite graphs is not finitely axiomatizable (that is, there is no finite theory $T$ such that the models of $T$ are exactly the infinite graphs).

- Suppose that $P$ is a unary predicate and $Q$ is a propositional variable. Give a formal proof of the following: $(\forall x(P(x)\to Q))\to((\forall xP(x))\to Q)$.
- Suppose that $R$ and $S$ are unary predicates. Show that there exists a formal proof of the following: $\forall x(R(x)\to S(x))\to(\exists x R(x)\to \exists x S(x))$.
- Kunen, exercise II.11.16. Suppose $R$ is a binary predicate and use the soundness theorem to show that there does not exist a formal proof of $\forall y\exists x R(x,y)\to\exists x\forall y R(x,y)$.
- Give an example of a structure $A$ and a formula $\phi(x)$ such that $A\models\exists x\phi(x)$ but there is no term $\tau$ such that $A\models\phi(\tau)$.

- Show that logical axiom 3 is valid.
- Show that relation defined by $\sigma\sim\tau$ if and only if $\Sigma\vdash\sigma=\tau$ is an equivalence relation.
- Kunen, exercise II.10.6.
- (2pts) Kunen, exercise II.11.11.
- Kunen, exercise II.11.14
- Kunen, exercise II.11.15

- Prove that the connectives $\vee$, $\wedge$, and $\leftrightarrow$ can all be defined using only $\neg$ and $\rightarrow$.
- Using the signature of ordered field theory, write well-formed sentences stating each of the following theorems.
- Every bounded set has a supremum.
- Every cubic polynomial has a root.
- The triangle inequality.

- Prove Kunen, Lemma II.5.4.

- (2pts) Write a computer program that takes a Polish lexicon and string of symbols as input, and determines whether the given string is a well-formed expression.

- Define $+$, $\times$, and $<$ on the rational numbers as they were constructed in class.
- Find the rank of the objects: $\mathbb N,\mathbb Z,\mathbb Q,\mathbb R,-2/3,\pi$. (See Kunen, exercise I.15.2 for reference.)
- Write each statement as a standard logical expression. Then develop a lexicon and convert it to prefix notation.
- The polynomial $x^4+3x+5$ has a root.
- Any natural $n$ can be written as the sum of four squares.

- Convert the expressions from prefix notation to standard logical notation. The arity function may be inferred from the standard meaning of the symbols, except: $x$ has arity $1$; $A$ has arity $1$ and means “absolute value”.
- $\forall a\mathord{\rightarrow}\mathord{\in}aS\mathord{\leq}ab$
- $\forall\epsilon\exists N\forall n\mathord{\rightarrow}\mathord{>}nN\mathord{<}A\mathord{-}xnL\epsilon$

- A linear order is said to be
*dense*Prove the following classical theorem: Any two countable, dense linear orders without endpoints are isomorphic to one another. - Define $+$, $\times$, and $<$ on the real numbers constructed in class.
- (2pts) Kunen, exercise I.15.10 (forget the last sentence).
- Kunen, exercise I.15.14.
- Kunen, exercise I.15.15.
- (2pts) Kunen, exercise II.4.7.

- Show that $(\kappa^{\lambda})^\mu=\kappa^{\lambda\mu}$ and $\kappa^{\lambda+\mu}=\kappa^\lambda\kappa^\mu$.
- Find the cardinality of the following sets:
- The set of finite subsets of $\omega_1$
- The set of countable subsets of $\omega_1$
- The set of functions whose domain is a countable subset of $\omega_1$ and whose range is contained in $\omega_2$
- The set of functions whose domain is a countable subset of $\omega_2$ and whose range is contained in $\omega_1$

- For any $\alpha$ the Axiom of Union holds in $V_\alpha$.
- For limit $\lambda$ the Axioms of Pairing and Power Set hold in $V_\lambda$.

- Kunen, exercise I.13.18. Prove that there is a cardinal $\alpha$ such that $\alpha=\beth_\alpha$.
- (2pts) The GCH is equivalent to the statement that $\kappa^{\mathrm{cf}(\kappa)}=\kappa^+$ for all cardinals $\kappa$.
- Kunen, exercise I.14.15. If $K$ is a proper class satisfying $y\subset K\implies y\in K$ then $WF\subset K$.
- (2pts) Kunen, exercise I.14.17. Prove a stronger version of the replacement axiom (see the text).
- Kunen, exercise I.14.23. Prove the version of the induction theorem known as $\in$-induction (see the text).

- Kunen, exercise I.11.31. Prove that the $\aleph$ sequence is strictly increasing and that every infinite cardinal is in the $\aleph$ sequence.
- Kunen, exercise I.11.33. Prove that there exists an $\aleph$-fixed point.
- Show that the Choice Set version of AC and the Choice Function version of AC are equivalent.
- Kunen, exercise I.12.13. (Without AC) There is a surjective function from $\mathcal P(\omega)$ to $\omega_1$.

- (2pts) Based on Kunen, exercise I.11.6, first part. Give a careful drawing of the graph of a bijection between [0,1] and [0,1/2). Include any helpful explanations with your drawing so it is clear what you mean.
- Kunen, exercise I.11.35. Prove there is an uncountable ordinal without using Replacement or Choice.
- Kunen, exercise I.11.36. Every countable strict total ordering is isomorphic to a subset of $\mathbb Q$.
- Kunen, exercise I.12.17(i). (Without AC) there is an injection from $\omega$ to $A$ iff $A$ is not Dedekind finite.
- Kunen, exercise I.12.17(ii). (With AC) Dedekind finite is equivalent to finite.

- The lexicographic product of two well-orders is again a well-order.
- Use the recursion theorem to show that the following are functions. (You may assume that you already know that $+,\times$ are functions.)
- $f(n)=$ the $n$th prime number
- $f(n)=2^n$
- $f(n)=n^{n^n}$

- Kunen, exercise I.11.3.

- Kunen, exercise I.9.6, parts (1)(2)
- Kunen, exercise I.9.6, parts (3)(4)
- Show that the cartesian product $A\times B$ can be constructed using the Power Set axiom instead of the Replacement axiom.
- (2pts) Is there an uncountable subset of $\mathbb R$ which is well-ordered by the usual ordering on $\mathbb R$?

- Which of the following rules hold for a function f?
- $f``(A\cup B)=f``A\cup f``B$
- $f``(A\cap B)=f``A\cap f``B$
- if $A\subset B$ then $f``A\subset f``B$

- Draw a graph of each of the following relations on $\mathbb R$:
- $R$ = <
- $xSy$ iff $x^2=y^2$
- $xTy$ iff $x^2=1-y^2$

- Kunen, exercise I.7.19. If $R$ is a finite relation, then $R$ is well-founded iff $R$ is acyclic.
- Kunen, exercise I.7.20. If $R$ is a well-order on $X$ and $A\subset X$, then the restriction of $R$ to $A$ is a well-order on $A$.
- Kunen, exercise I.8.11. If $\alpha$ is an ordinal then $S(\alpha)$ is an ordinal; and $\alpha<S(\alpha)$; and $\gamma<S(\alpha)$ iff $\gamma\leq\alpha$.

- Kunen, exercise I.7.13. The lexicographic product of (strict) linear orders is a strict total order.
- Kunen, exercise I.7.23. The lexicographic product of two well-orders is again a well-order.
- If $R,S,T$ are arbitrary relations show that $(R\circ S)\circ T=R\circ(S\circ T)$.
- If $f$ and $g$ are functions, show that $f\cap g$ is a function. Under what circumstances will $f\cup g$ be a function?
- Let $\mathcal F$ is a family of functions such that for all $f,g\in F$ we have $f\cup g$ is a function. Show that $\bigcup\mathcal F$ is a function.
- Show that the class of all ordered pairs is not a set.
- Kunen, exercise I.7.16. Suppose $[x,y]$ is an arbitrary pairing function and $R$ is a set of such pairs. Then the domain $\{x:\exists y [x,y]\in R\}$ is a set.
- (2pts) Let $A$ and $B$ be finite
*disjoint*sets, and consider the following game. Two players alternate playing an element $\langle a,b\rangle\in A\times B$ subject to the condition that $a$ and $b$ may not*both*have been used already. The game ends when one player is left without a legal move, and that player is the loser. What is an upper bound on the number of moves in this game? Which player has a winning strategy?

- Kunen, exercise I.6.3. Find a model of extensionality plus there is no empty set.
- Kunen, exercise I.6.15. If $\langle x,y\rangle=\langle x’,y’\rangle$ then $x=x’$ and $y=y’$.
- Decide whether each of the following alternative definitions of $\langle x,y\rangle$ is a good one:
- $\langle x,y\rangle= ${ x, {y} }
- $\langle x,y\rangle=$ { x, {x,y} }
- $\langle x,y\rangle=${ {x}, { {y} } }

- Kunen, exercise I.6.11 and I.6.13.
- The axiom of comprehension begins $\exists y\ldots$. Show that in fact this $y$ is unique.
- Show that the naive comprehension axiom (I.6.4) is false in the instance when $Q(x)$ is $\neg\exists u(x\in u\wedge u\in x)$.
- Generalize the previous problem further.

- Interpret each of the symbolic expressions as English mathematical statements.
- $(\forall N\in\mathbb N)(\exists n>N)(\forall a,b)(n=ab\implies n=a\vee n=b)$
- $S\subset\mathbb R\wedge(\exists x\in S)(\forall y\in S)(y\leq x)$

- Write each of the English mathematical statements as symbolic expressions.
- The sum of any two odd numbers is even.
- Every real cubic polynomial has a real root.

- Recall that the ``exclusive or’’ connective is written $P\oplus Q$ and is equivalent to $(P\wedge\neg Q)\vee(\neg P\wedge Q)$.
- Write a truth table for $P\oplus Q$.
- Draw a Venn diagram for $P\oplus Q$.
- Is $\oplus$ associative? Prove your answer.

- Kunen, exercise I.2.1

- Write the statement more formally and prove it: “The empty set is unique.”
- Find the definition of the $\cap$ operation on page 10. Give a definition of the $\cup$ and $\smallsetminus$ (set difference) operations. Then prove the De Morgan law: $x\smallsetminus(y\cup z)=(x\smallsetminus y)\cap(x\smallsetminus z)$.
- Prove by induction: if $A$ is a set with $n$ elements, then $A$ has exactly $2^n$ subsets.
- Show that Set Existence axiom follows from the other axioms.
- (2pts) Show that the Pairing axiom follows from the other axioms.
- (2pts) Show that the Separation (Comprehension) axiom follows from the other axioms.