Lecture Notes

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1. Numbers, Sets, and Sequences

Rational Zeros Theorem. For polynomials of the form c_nxⁿ + … + c₀ = 0 , where each coefficient is an integer, then the only rational solutions have the form $\frac{c}{d}$ where c divides c_n and d divides c₀; rational root r must divide c₀.

The maximum of a set S is the largest element in the set.
The minimum is the smallest element in the set.
The $\inf$ of S is the greatest lower bound.
The $\sup$ of S is the smallest upper bound.
S is bounded if $\forall$s $\in$ S, s$\leq$M for some M $\in$ $\reals$
Completeness Axiom. If S is a nonempty bounded set in $\reals$, then $\inf$ S and $\sup$ S exist.
Archimedean Property. If a, b $\gt$ 0, then $\exists$n such that na $\gt$ b.

A sequence (s_n) is a function mapping from $\N$ to $\R$. It converges to s if $\forall$ $\epsilon$ > 0 there exists N such that N > n $\implies$ |(s_n)-s| < $\epsilon$
In other words, $\lim$(s_n) = s

Important limit theorems include:
$\lim$(s_n)(t_n) = ($\lim$(s_n))($\lim$(t_n))
$\lim$(s_n)+(t_n) = ($\lim$(s_n)) + ($\lim$(t_n))
$\lim$($\frac{1}{n^p}$) = 0 for p > 0
$\lim$ n^(1/n) = 1

A subsequence (s_n(k)) of (s_n) is a sequence that is a subset of the elements in the original sequence with relative order preserved.
Bolzano-Weierstrass Theorem. Every bounded sequence has a convergent subsequence, having some subsequential limit.

Given any (s_n) and let S be the set of subsequential limits of (s_n). Define:
$\lim$ $\sup$ (s_n) = $\lim\limits_{N \to \infin}$ $\sup${(s_n): n > N} = $\sup$ S
$\lim$ $\inf$ (s_n) = $\lim\limits_{N \to \infin}$ $\inf${(s_n): n > N} = $\inf$ S

$\lim$ $\inf$ |s_n+1| / |s_n| $\leq$ $\lim$ $\inf$ |s_n|^(1/n) $\leq$ $\lim$ $\sup$ |s_n|^(1/n) $\leq$ $\lim$ $\sup$ |s_n+1| / |s_n|

2. Topology

Metric Space: A set S with a metric, distance function d. For any x,y,z $\in$ S
(1) d(x,y) > 0 if x $\not =$ y, d(x,x) = 0
(2) d(x,y) = d(y,x)
(3) d(x,z) $\leq$ d(x,y) + d(y,z)
Important* A metric is only valid if it outputs a real number for any inputs, ie. d(x,y) = $\infin$ is not valid.

A limit point p of a set S is such that for some $\epsilon$ radius ball around p, there exists an element q $\not =$ p such that q $\notin$ S. Note that limit points may or may not lie in the set.
A set S is open if for every point p in S is interior in S. Think open ball of $\epsilon$ radius in S centered at p.
A set S is closed if every limit point of S is a point in S.
A set S is perfect if it is closed and every interior point is a limit point.
A set S is dense in a metric space X if every point in X is either a limit point of S or in S itself.
The closure of a set S is the union of S and the set of its limit points. It can also be thought of as the intersection of all closed sets containing S.

An open cover for S is a collection of open sets that covers S.
A set S is compact if for all open covers {G} of S, there exists a finite subcover of {G} that covers S.
Heine-Borel Theorem. A subset E of $\R$ⁿ is compact if and only if it is closed and bounded.

A set S is connected if it cannot be written as disjoint union of nonempty, open sets

3. Series

A series converges if its partial sum $\Sigma$ s_n = M for some M $\in$ $\reals$.
A series $\Sigma$ s_n satisfies is Cauchy if for any $\epsilon$ > 0, there exists N such that n $\geq$ m > N $\implies$ |$\displaystyle\sum_{i=m}^n$ s_i| < $\epsilon$
If $\Sigma$ s_n converges, then $\lim$ s_n = 0. (Note this is a one-way statement)

Comparison Test
If $\Sigma$ a_n converges and |b_n| $\leq$ a_n $\forall$n, then $\Sigma$ b_n converges.
If $\Sigma$ a_n = $\infin$ and |b_n| $\geq$ a_n $\forall$n, then $\Sigma$ b_n = $\infin$.

Ratio Test $\Sigma$ a_n of nonzero terms
(i) converges if $\lim$ $\sup$ |a_n+1 / a_n| < 1
(ii) diverges if $\lim$ $\inf$ |a_n+1 / a_n| > 1
(iii) else inconclusive test

Root Test Let $\alpha$ = $\lim$ $\sup$ |a_n|^(1/n). Then $\Sigma$ a_n
(i) converges if $\alpha$ < 1
(ii) diverges if $\alpha$ > 1
(iii) else inconclusive test

Alternating Series Theorem If a_n is monotonically decreasing and $\lim$ a_n = 0, then $\Sigma$ (-1)ⁿ a_n converges.

4. Continuity and Convergence

There are three main definitions for a continuous function $\bold{f}$:
(1) $\bold{f}$ continuous at x if for each $\epsilon$ > 0, there exists $\delta$ > 0 such that |x-y| < $\delta$ where y $\in$ domain(f) $\implies$ |f(x) - f(y)| < $\epsilon$
(2) $\bold{f}$ continuous at x if for all sequences (s_n) in domain(f) that converge to x, $\lim$ f(s_n) = f(x)
(3) Let $\bold{f}$ be mapping between metric spaces X $\to$ Y. $\bold{f}$ is continuous if the preimage $\bold{f}$^-1 of any open set in Y is open in X. Similarly if the preimage $\bold{f}$^-1 of any closed set in Y is closed in X.

A function $\bold{f}$ is uniformly continuous if for all x in domain(f), for each $\epsilon$ > 0, there exists $\delta$ > 0 such that |x-y| < $\delta$ where y $\in$ domain(f) $\implies$ |f(x) - f(y)| < $\epsilon$.

Generally, a continuous function $\bold{f}$ sends a compact set to another compact set. In this case, $\bold{f}$ is bounded, and $\sup$ $\bold{f}$ and $\inf$ $\bold{f}$ exists.
A continuous function acting on a compact set is uniformly continuous on this interval.
Cauchy relation: If $\bold{f}$ is uniformly continuous on a set S and s_n is a Cauchy sequence in S, then $\bold{f}$(s_n) is a Cauchy sequence.

Generally, a continuous function $\bold{f}$ sends connected set to another connected set.

Intermediate Value Theorem. Let $\bold{f}$ be a continuous real function on interval [a,b]. Then for all y between f(a) and f(b), there exists at least one x $\in$ (a,b) such that f(x) = y.

If a function $\bold{f}$ is discontinuous at x, and both $\bold{f}$(x+) and $\bold{f}$(x-) exist, then $\bold{f}$ is said to have discontinuity of the first kind at x, or simple discontinuity.

A function $\bold{f}$_n converges to f pointwise if for each x in the domain, $\lim\limits_{n \to \infin}$ $\bold{f}$_n(x) = f(x).

A function $\bold{f}$_n converges to f uniformly if $\lim$ $\sup${|$\bold{f}$_n - f|} = 0.
In other words, for any fixed $\epsilon$ > 0 there exists N such that n > N $\implies$ |$\bold{f}$_n(x) - f(x)| < $\epsilon$ for all x in the domain. This is a stronger restraint than pointwise convergence, where we only cared about each x individually. (Compare with regular vs uniform continuity)