Date

What we discussed/How we spent our time

Aug 26

Syllabus. Text.
Some discussion about the foundations of mathematics.
The idea of a set. The axioms.
The directed graph model of set theory.
We explained the meaning of the axioms of
Extensionality, Empty Set, Infinity,
Pairing, and Union.

Aug 28

We discussed the language of set theory.
We then discussed meaning of the axioms of Power Set,
Comprehension, Replacement, and Foundation.
We also discussed the Russell set and
the explained why naive set theory is inconsistent.

Aug 30

We discussed the meaning of the Axiom of Choice.
We discussed the fact that we may
form the intersection of any nonempty class of sets,
but we can only form the union of a set of sets.
We practiced writing the axioms formally.

Sep 4

We gave the Kuratowski definition of ordered pairs,
$(a,b)=\{\{a\},\{a,b\}\}$,
and proved the characteristic property of ordered pairs.
We defined ordered triples and $n$tuples.
We defined $A\times B$ and showed that it is a set
(if $A$ and $B$ are). We defined relations and functions.
We gave examples of different kinds of relations
in mathematics (equality, $\in$, order, betweenness,
adjacency, graphs of functions and operations).

Sep 6

We discussed terminology for functions.
We defined class functions.

Sep 9

We discussed the relationship between
coimages/partitions and kernels/equivalence relations.
Quiz 1!

Sep 11

We discussed strict and nonstrict orders.
Terminology: comparable/incomparable, maximum/minimum,
maximal/minimal, linear order/total order/chain,
antichain, wellfounded/wellordered.
We sketched a proof that wellordered = wellfounded+linearly ordered.

Sep 13

We discussed the principle of induction.
We worked on this handout.

Sep 16

We discussed the principle of strong induction.
We stated the Recursion Theorem.
Quiz 2!

Sep 18

We began to discuss the arithmetic
of $\mathbb N$, following
this handout.
Some hints!

Sep 20

We discussed more arithmetic
of $\mathbb N$ (distributive law, associative law for multiplication).
We discussed the meaning of ``Logic Is Backwards''.
We sketched the proof of the Recursion Theorem (pages 4749).

Sep 23

We discussed generalizations of the Recursion
Theorem: the parametrized version, the strong version,
and the version with partial functions.
Quiz 3!

Sep 25

We began to discuss ordinal and cardinal numbers.
Today
we discussed the meaning of
$A=B$
$A\leqB$
finite
infinite
countably infinite
countable
uncountable
We sketched the proof of the CantorBernsteinSchroeder Theorem.

Sep 27

We continued discussing ordinal and cardinal numbers,
and also mentioned the von Neumann hierarchy.
$n\in V_{n+1}$, $n\notin V_{n}$.
$V_{\omega}$ = hereditarily finite sets is a model of ZFC  Infinity.
$V_{\omega+\omega}$ = is a model of ZFC  Replacement.
Defined transitive sets, ordinals.
Stated the wellordering theorem.
defined cardinal numbers ( = initial ordinals).
Argued that equipotence classes of ordinals are bounded intervals.

Sep 30

We proved Cantor's Theorem.
We introduced $\aleph$ numbers.
Quiz 4!

Oct 2

We discussed why the sum rule and product rule
satisfy the recursion for $+$ and $\cdot$ on $\omega$.
Then we discussed why equipotence classes of finite
ordinals are singletons, why any subset of a finite
set is finite, and why $\omega$ is infinite.

Oct 4

We discussed:
$X\cup Y\leq X+Y$.
A finite union of finite sets is finite.
$X$ finite implies ${\mathcal P}(X)$ finite.
$X$ infinite implies $X>n$ for each $n\in\omega$ (without AC).
An infinite subset of a countably infinite set is countably infinite.
$X\leq \omega$ implies $X=\omega$ or $X=k$ for some $k\in\omega$.
A finite union of countable sets is countable (without AC).
Midterm Review!.

Oct 7

We discussed:
If $A$ is countably infinite and
$f:A\to B$ is surjective, then $B$ is finite
or $B=\omega$.
$\omega\times\omega=\omega$.
If $A=B=C=\omega$, then
$A=B\times C=\omega$.
If $A=\omega$, then $A=A^n$ for any $n\in\omega$.
A countable union of countable sets is countable.
(In ZFC, the statement stands as given. In ZF, the statement
stands provided we are given a sequence of
enumerations $\langle a_0, a_1,\ldots\rangle$
for the sequence $\langle A_0, A_1,\ldots\rangle$ of sets.)
Quiz 5!

Oct 9

Midterm Review!.

Oct 11

Midterm!.

Oct 14

We proved that the set of finite strings in a
nonempty countable alphabet is countably infinite.
We derived some corollaries, the most of important
of which are
${\mathcal P}_{\textrm{fin}}(\omega)=\omega$.
There are only countably many algebraic numbers.
No Quiz!

Oct 16

We defined addition, multiplication,
and exponentiation of cardinal numbers.
We discussed some rules of cardinal arithmetic.
We argued that $\mathbb R=2^{\aleph_0}$.
We argued that the number of continuous functions
$f:\mathbb R\to \mathbb R$ is $2^{\aleph_0}$, while
the number of all functions $g:\mathbb R\to \mathbb R$
is the strictly larger value
$\left \mathbb R^{\mathbb R}\right=2^{2^{\aleph_0}}$.

Oct 18

We argued that the following sets have size $2^{\aleph_0}$:
the set of irrational real numbers,
the set of infinite subsets of $\omega$,
the set of permutations of $\omega$.
We recalled the definition of ordinal number
and wellordered set. We proved that if
$f: \mathbb W\to \mathbb W$ is a
$<$homomorphism from a wellordered set
$\mathbb W$ to itself,
then it must be increasing in the sense that
$(\forall x)(f(x)\geq x)$ holds.

Oct 21

We discussed Theorem 6.1.3 and Corollary 6.1.5.
Quiz 6!

Oct 23

We worked through almost all of Section 2 of Chapter 6.
Our ultimate goal was to prove that the set ON
of ordinal numbers is a proper class that is well
ordered (in the class sense) by $\in$.

Oct 25

We proved that ON is a proper class that is well
ordered (in the class sense) by $\in$.
We discussed Russell's Paradox, Cantor's Paradox, and
the BuraliForti Paradox.

Oct 28

We discussed transfinite
induction and recursion.
We defined ordinal arithmetic.
We defined what it means for a function defined
on the ordinals to be continuous.
Quiz 7!

Oct 30

We discussed how to visualize the arithmetic operations on the ordinals.
We also defined the ``order type''
of a wellordered set, and the ``lexicographic'' and
``antilexicographic'' orderings of products and
sets of strings.

Nov 1

We discussed a transfinite induction proof.
We briefly mentioned the idea behind Cantor Normal Form
and the definition of $\varepsilon_0$.
Finally we discussed the definition of alephs.

Nov 4

We discussed $\aleph$ numbers and the possible meaning
of ``cardinality'' in the absence of the axiom of choice.
We got off onto a side discussion of the question:
Is it true that ``$\omega\leq A$ when $A$ is infinite''
in the absence of choice?
Quiz 8!

Nov 6

We reviewed the quiz.
We defined the Hartogs number of a set.
We showed that the Hartogs number of a set, if it exists, is an
initial ordinal (an aleph number).
Using the Axiom of Replacement, we showed that every
set has a Hartogs number. We finished the lecture by proving the
following technical lemma, which will be used to prove
Tarski's Theorem:
Lemma. (ZF) If $H$ is a wellorderable set that is disjoint
from some set $A$, and $A\times H = A\cup H$, then either
$A\leq H$ or $H\leq A$.

Nov 8

We proved (in ZF) that if every infinite set
satisfies $B\times B=B$, then every infinite
set is wellorderable.
We began a proof of the converse, but did not finish.

Nov 11

We finished the proof that if every set is wellorderable,
then any infinite set $B$ must satisfy $B\times B=B$.
We remarked that an ordinal $\alpha$ has the property
that the canonical wellordering of $\alpha\times\alpha$
has order type $\alpha$ iff $\alpha = 0, 1$ or
$\alpha = \omega^{\omega^\beta}$ for some ordinal $\beta$.
We derived from the main theorem
that if $\lambda$ and $\kappa$ are finite cardinals or alephs
and at least one is infinite, then
$\lambda+\kappa = \lambda\cdot\kappa = \max(\lambda,\kappa)$.
Quiz 9!

Nov 13

We discussed
this handout
on the Axiom of Choice.
We explained the equivalence of (1), (2), (3), (4), (5),
and we proved that (14) implies (1).

Nov 15

We worked on the proof that AoC $\equiv$ WellOrdering Theorem $\equiv$
Zorn's Lemma.

Nov 18

We finished the proof that AoC $\equiv$ WellOrdering Theorem $\equiv$
Zorn's Lemma.
Quiz 10!

Nov 20

We showd the AC is equivalent to the statement
that any two sets have comparable cardinality.
We sketched two proofs that every vector space
has a basis, one using the WellOrdering Theorem
and the other using Zorn's Lemma.

Nov 22

We started discussing cardinal arithmetic
by defining infinite sum and product. We also
mentioned the relation between $\leq$ and $\leq_*$.
We showed that
$\sum_{\alpha<\lambda} \kappa_{\alpha} = \kappa\cdot\lambda$
if $\kappa_{\alpha}>0$ for all $\alpha$
and if $\kappa := \sup(\kappa_{\alpha})$. We proved
König's Theorem, from which we derived Cantor's Theorem
(and claimed that $\kappa<{\kappa}^{\textrm{cf}(\kappa)}$).

Dec 2

We defined cofinality, and proved some basic things
about the cofinality of ordinals and cardinals, such as
$\textrm{cf}(\alpha)\leq \alpha$.
$\textrm{cf}(0) = 0$, hence $0$ is regular.
Every successor ordinal has cofinality $1$, hence
$1$ is the only regular successor ordinal.
Every infinite limit ordinal has cofinality $\geq \omega$.
In particular, $\omega$ is regular.
$\textrm{cf}(\aleph_{\omega}) = \omega$, so $\aleph_{\omega}$ is singular.
$\textrm{cf}(\textrm{cf}(\alpha))=\textrm{cf}(\alpha)$.
So, if $\alpha$ is infinite, then $\textrm{cf}(\alpha)$ is an
infinite, regular cardinal.
Any cofinal subset of an ordinal $\alpha$ contains
a cofinal subset of cardinality $\textrm{cf}(\alpha)$.
An infinite cardinal
$\kappa$ is a (disjoint) union of $\lambda$many
subseteq of size $<\kappa$ iff $\textrm{cf}(\kappa)\leq\lambda$
Notes on Cardinal Arithmetic!
No Quiz!

Dec 4

We defined the continuum function $C(\kappa) = 2^{\kappa}$,
the aleph function $\aleph_{\alpha}$, the
beth function $\beth_{\alpha}$, and the gimel function
$\gimel(\kappa) = \kappa^{\textrm{cf}(\kappa)}$.
We stated CH ($\aleph_1=\beth_1$)
and GCH ($\aleph_{\alpha}=\beth_{\alpha}$ for all $\alpha$)
in terms of the functions $\aleph$ and $\beth$.
We reviewed some elements of the
Cardinal Arithmetic
handout. We explained why Theorems 2, 3, 4 and
Corollary 5 are true.

Dec 6

Samuel recited the following poem he wrote!
There was a time, when we were young,
we thought infinity
was never wider than the sky,
nor deeper than the sea.
But now we've gazed into the depths
of "omega" and beyond;
and sit, in dumbstruck silence, now,
our innocence all gone.
So let us not attempt to see
what must remain unseen;
let us rejoice in finite things,
in one, and two, and three:
For if we crack and crush our minds
with things too huge to be known,
we'll end up deranged, and lost in our brains,
as Cantor and Gödel have shown.
So have no fear, O students, professors:
infinity cares for herself.
Our work is to care for the simpler things;
let us do so, that we may be well.
Then we proved that successor cardinals are regular.
We established that $\textrm{cf}\left(2^{\kappa}\right)>\kappa$.
We discussed Easton's Theorem.
We proved the first two cases of the
Main Theorem of Cardinal Arithmetic.
(These are the cases where $\lambda$ is large
relative to $\kappa$ or very small relative to $\kappa$.)
Review Sheet for Final!.

Dec 9

No Quiz!
We finished the proof of the
Main Theorem of Cardinal Arithmetic.
We pointed out that if
$V$ is a
$\lambda$dimensional
vector space (where $\lambda$ is infinite)
over a field $\mathbb F$ of cardinality
$\kappa$,
then $V^* = \dim(V^*) = \kappa^{\lambda}$.
Here $V^*$ is the dual space of $V$.
(This is a ``reallife'' situation where one might need to
compute $\kappa^{\lambda}$.)

Dec 11

We
reviewed for the final exam.
