Sequences and Series

Sequences

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A sequence $(a_n)$ is an infinite list of numbers $(a_0, a_1,...)$. We can define sequences through nth term rules or recursively.

Convergent Sequences

A sequence $a_n$ is said to converge to a limit $l \in \mathbb{R}$ if for every small $\epsilon > 0$ there is a related integer $N$ such that $|a_n - l| < \epsilon \; \forall n > N$. This is denoted as

In layman's terms, if a sequence converges, then for every (increasingly small) positive number, the difference between the sequence's terms and the limit will be eventually smaller.

$N$ here is like a threshold indicator, where every term of the sequence after $N$ will be within the $\epsilon$ difference.

We generally worth with $n \in \mathbb{N}$.

Example. The sequence $a_n = \frac{1}{n}$ converges to 0. We can prove this by going to the definition - let us have a small $\epsilon > 0$.

Then $\exists N : a_N - 0 < \epsilon$, or $\frac{1}{N} < \epsilon$. We can choose any integer $N > \frac{1}{e}$, to demonstrate that this is possible for any $\epsilon$. For any $n > N$: $|\frac{1}{n} - 0| = \frac{1}{n} < \frac{1}{N} < \epsilon,$ to write it out as an equation.

If possible, try break a large sequence down into simpler components. They can be combined using the following:

Combination Rules for Convergent Sequences

For convergent sequences $a_n \rightarrow \alpha, b+n \rightarrow \beta, c_n \rightarrow \gamma$:

1. Sum rule: $a_n + b_n \rightarrow \alpha + \beta$
2. Scalar multiple rule: $\lambda a_n \rightarrow \lambda\alpha, \; \lambda \in \mathbb{R}$
3. Product rule: $a_n b_n \rightarrow \alpha\beta$
4. Reciprocal rule: $\frac{1}{a_n} \rightarrow \frac{1}{\alpha}$
5. Quotient rule: $\frac{b_n}{a_n} \rightarrow \frac{\beta}{\alpha}$
6. Hybrid rule: $\frac{b_n c_n}{a_n} \rightarrow \frac{\beta\gamma}{\alpha}$

Exercise

Show that the following sequence converges, find its limit: $a_n = \frac{(n+2)(2n-1)}{3n^2 + 1}$

Solution. The components do not converge themselves, a technique here is dividing by the fastest increasing term. Dividing by $n^2$, $a_n = \frac{\frac{(n+2)}{n}\frac{(2n-1)}{n}}{\frac{3n^2 +1}{n^2}} = \frac{(1 + \frac{2}{n})(2 - \frac{1}{n})}{3 + \frac{1}{n^2}}.$

Now the components $\frac{1}{n}$ and $\frac{1}{n^2}$ converge to 0, so applying combination rules we can get $a_n \rightarrow \frac{(1+0)(2-0)}{3+0} = \frac{2}{3}.$

Bounded sequences

A sequence $a_n$ is bounded above if there is a number $U : a_n \leq U \forall n$. $a_n$ is bounded below if there is a number $L : L \leq a_n \forall n$. A sequence is bounded if it is bounded both above and below.

Subsequences

A subsequence of a sequence is obtained from the original sequence by deleting some terms. We can say $a_{2n}, a_{2n+1}$ are the even and odd subsequences respectively of $a_n$.

A sequence $a_n$ is an increasing sequence if $a_{n+1} \geq a_n \;\forall n$. It is a decreasing sequence if $a_{n+1} \leq a_n \;\forall n$.

Basic Properties of Convergent Sequences

1. A convergent sequence has a unique limit.
2. If $a_n \rightarrow l$ then every subsequence of $a_n$ also converges to $l$.
3. If $a_n \rightarrow l$ then $\|a_n\| \rightarrow \|l\|$.
4. The squeeze rule. If $a_n \rightarrow l, b_n \rightarrow l; a_n < c_n < b_n \; \forall n$ then $c_n \rightarrow l$.
5. A convergent sequence is always bounded.
6. An increasing sequence which is bounded above converges. A decreasing sequence which is bounded below converges.

The sequence $c_n$ is "squeezed" between two sequences which both converge to the same limit, so naturally it will too.$\exists B > 0 : - B \leq a_n \leq B, \; \forall n$.

You can demonstrate an alternating sequence (e.g. $(-1)^n$) doesn't converge by looking at subsequences.

Exercise

Show that for $x \geq 0, n > 0$, this: $(1+x)^{\frac{1}{n}} \leq 1 + \frac{x}{n}$.

Hence deduce/show that if $c > 0$ then $c^\frac{1}{n} \rightarrow 1$.

Reminder: the binomial theorem is $(1 + x)^n = 1 + nx + \frac{n(n-1)}{2!}x^2 + ... + \frac{n!}{k!(n-k)!}x^k$

Solution. Firstly, we can rearrange and use the binomial theorem.

Thus if we take $n^\textrm{th}$ roots of each side we will demonstrate that $(1+x)^\frac{1}{n} \leq 1 + \frac{x}{n}.$

For the second part, we need to consider separately the cases $c \geq 1$ and $c < 1$.

If $c \geq 1$ then $c^\frac{1}{n} \geq 1$. If we use the inequality we demonstrated in the first part, letting $x = c-1 \geq 0$ we get $1 \leq c^\frac{1}{n} \leq 1 + \frac{c-1}{n}.$

By the squeeze rule, we can (in an ideal world) see that $c^\frac{1}{n} \rightarrow 1$.

Finally if $c < 1$ then $\frac{1}{c} > 1$ and by the reciprocal rule $\frac{1}{c^\frac{1}{n}} \rightarrow 1$.

Did you get that? Me neither.

Divergent Sequences

A sequence $a_n$ is said to diverge to infinity if $\forall K \in \mathbb{R}\; \exists N : n > N \implies a_n > K$.

In plain English, there is a point in $a_n$ where the terms are greater than any real number one picks.

We denote this as $a_n \rightarrow \infty$. $a_n$ diverges to $-\infty$ if $-a_n \rightarrow \infty$.

A divergent sequence that doesn't go off to infinity is said to oscillate.

Basic convergent sequences

Series

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A series $\sum a_n$ is a pair of sequences:

1. A sequence $a_n$ called the sequence of terms
2. A sequence $s_n$ called the sequence of partial sums defined as $s_n = \sum_{k=0}^{n} a_k.$

If the sequence of partial sums converges to $s$, then the series converges to the sum $s$; $\sum_{n=0}^{\infty} a_n = s.$ Divergent if not.

A standard one is the geometric series, $\sum r^n$ which will always converge to $\frac{1}{1-r}$ when $|r| < 1$. You can prove this by working out $rs_n - s_n$.

And $|r| < 1$ means that $r$-power will converge to 0.

A standard divergent series is the harmonic series, $\sum \frac{1}{n}$. You can prove this by estimating partial sums $s_2, s_4, s_8, s_{16}, ...$ and see that it will forever build to $1 + 0.5 + 0.5 + 0.5 + ...$. $s_{2^n} > 1 + \frac{n}{2}.$ Though any series $\sum \frac{1}{n^k}$ where $k > 1$ will always converge.

Basic Properties of Convergent Series

1. Sum rule, $\sum a_n$ converges to $s,\; \sum b_n$ converges to $t \implies \sum (a_n + b_n)$ converges to $s + t$.
2. Multiple rule, $\sum a_n$ converges to $s, \; \lambda \in \mathbb{R} \implies \sum \lambda a_n$ converges to $\lambda s$.
3. $\sum a_n$ converges $\implies a_n \rightarrow 0$.

4. $$\sum

   a_n

   $converges$\implies \sum a_n$$ also converges.

No. 4 is only useful really when you have an "oscillating" series, like $\sum \frac{(-1)^n}{2^n}$.

The Comparison Test

Suppose that $0 \leq a_n \leq b_n$ for all $n$,

1. If $\sum b_n$ converges then so does $\sum a_n$
2. If $\sum a_n$ diverges then so does $\sum b_n$.

Usually with comparison test questions you just pull out equations from thin air, but (especially with rational functions) there's usually a technique for doing so based on the largest power of $n$, and slowly chipping away a piece at a time until we get a very simple expression.

Exercises

Determine whether the following series converges or diverges:

1. $$\sum \frac{n+2}{n^3 - n^2 + 1}$$
2. $$\sum \frac{n^2 + 4}{2n^3 - n + 1}$$

Solution 1. Take a look at the fraction $\frac{n+2}{n^3 - n^2 + 1}$. If we want to find a smaller fraction, we want a smaller numerator and a larger denominator than our current fraction.

So we get the fraction $\frac{n}{n^3}$ or $\frac{1}{n^2}$ which we know will converge as a series. No help there.

For a larger fraction, we want a bigger numerator and a smaller denominator;

Admittedly, at the end we were pulling expressions out of thin air a bit but hopefully you can see that it's all valid. Thus $\frac{n+2}{n^3 - n^2 + 1} \leq \frac{6}{n^2}$ and since we know $\sum \frac{6}{n^2}$ converges ($6 \sum \frac{1}{n^2}$) the original one must too.

Solution 2. We're gonna be cheat-y and say that this sequence does diverge. To show this, let's find a smaller fraction which diverges.

(the $-n$ more than cancels out the $+1$) and we can see that this gives us $\frac{1}{2n} \leq \frac{n^2 + 4}{2n^3 -n+1}$ And oh no, $\sum \frac{1}{2n}$ diverges (multiple rule) so the original one must too.

The Ratio Test

1. $0 \leq L < 1 \implies \sum a_n$ converges.
2. $L > 1$ or $L \textrm{ is } \infty \implies \sum a_n$ diverges.
3. $L = 1 \implies$ the test is inconclusive.

Useful in dealing with factorials.

Basic Convergent Series

• $\sum_{n=0}^{\infty} r^n = \frac{1}{1-r}$ for all $r : \|r\| < 1$
• $\sum \frac{1}{n^k}$ converges for all $k > 1$
• $\sum n^k r^n$ converges for $k > 0 \land \|r\| < 1$
• $\sum_{n=0}^{\infty} \frac{c^n}{n!} = e^c$ for all $c \in \mathbb{R}$

Basic Divergent Series

• $\sum \frac{1}{n^k}$ diverges for all $k < 1$.

Power series

A power series is one of the form $\sum a_n x^n$. Usually we start at $n = 0$, such that the sequence goes $a_0, a_1x, a_2x^2, ...$

Basic Properties of Power Series

Let

For positive $R_1, R_2$, and let $R = \min(R_1, R_2)$. Then for all $x \in (-R , R)$,

1. If $f(x) = g(x)$ then $a_n = b_n$ for all $n$
2. Sum rule, $f(x) + g(x) = \sum_{n=0}^{\infty} (a_n + b_n) x^n$
3. Multiple rule, $\lambda f(x) = \sum_{n=0}^{\infty} \lambda a_nx^n \; \forall \lambda \in \mathbb{R}$
4. Product rule, $f(x)g(x) = \sum_{n=0}^{\infty} (a_0b_n + a_1b_{n-1} + ... + a_{n-1}b_1 + a_nb_0)x^n$.

This also gives the general binomial theorem, which for any $q \in \mathbb{Q}, x \in (-1, 1)$

Radius of convergence

Lemma. If $\sum a_n R^n$ converges for some $R \geq 0$, then $\sum a_n x^n$ converges $\forall x : \|x\| < R$. (proof in notes)

$R \geq 0$ is the radius of convergence of a power series $\sum a_n x^n$ if it converges according to the above, and diverges if $|x| > R$. If a series converges $\forall x$ then the radius is infinity.

If the series $\sum a_n x^n$ has a conv. rad $R$ then it defines a function

You can find the radius of convergence using ratio test, essentially evaluate $\lim_{n \rightarrow \infty} \|\frac{a_{n+1}}{a_n}\| < 1.$ and you will get an $\|x\| < ...$ where that something is your radius.

Addendum: Partial Fractions

Partial fractions can be important in series.

Where a rational function is decomposed into a sum of simpler fractions, such as $\frac{cx+d}{(x-a)(x-b)} = \frac{A}{x-a} + \frac{B}{x-b}.$

When there is no repeated factor, simply substituting $x = a, x=b$ can eliminate a term and make it easy to find the unknowns $A, B$. When there is however, we need to just pick values of $x$ and solve from there.

If there is a repeated factor $(x-a)^n$, the partial sum will have all the fractions with denominations $(x-a), (x-a)^2,...$ up to $(x-a)^n$

If the degree of the numerator is higher than the denominator, then we have to divide out the numerator (polynomial long division) to get a valid separable fraction, $\frac{x^3 + 3x}{(x+1)(x-3)} = x+2 + \frac{10x+6}{(x+1)(x-3)}.$

Recurrences

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Like the differential equations of sequences.

A recurrence is a sequence defined, funnily enough, recursively.

For example, the Fibonacci sequence $F_n$ is defined as $F_n = F_{n-1} + F_{n-2}$ $F_0 = 0, F_1 = 1.$

The Towers of Hanoi game's number of steps taken was computed to be the recurrence $T_n = 2T_{n-1} + 1.$ For $T_0 = 0$ this gives $0, 1, 3, 7, 15, ...$ which we can see is $T_n = 2^n - 1$. This latter form is the closed form of $T_n$ and allows for immediate computation.

We are looking at solving linear recurrences with constant coefficients, of the form $x_n + a_1x_{n-1} + ... a_kx_{n-k} = f(n)$ where $f$ is a given function. (If the terms from 1 to k are given, then this describes a unique sequence.)

Homogenous recurrences

More importantly, we will look at homogenous ($f(n) = 0$) recurrences with $k = 2$. Thus, we want the general solution of $x_n + ax_{n-1} + bx_{n-2} =0.$

In the general case, we want to look for solutions of the form $A\lambda^n$ where $\lambda, A$ are constants. A bit of deriving later, we can get the auxiliary equation in lambda.

The auxiliary equation of the recurrence $x_n + ax_{n-1} + bx_{n-2} = 0$ is $\lambda^2 + a\lambda + b = 0.$

General Solution. of the recurrence $x_n + ax_{n-1} + bx_{n-2} = 0$,

Let $\lambda_1, \lambda_2$ be the roots of the auxiliary equation.

• $$\lambda_1 \neq \lambda 2 \implies x_n = A\lambda_1^n + B\lambda_2^n$$
• $$\lambda_1 = \lambda_2 \implies x_n = A\lambda_1^n + Bn\lambda_2^n$$

For constants $A, B$, which can be found if the first two terms of the sequence are known.

Exercise

Find a closed form for the Fibonacci sequence.

Solution. We have $F_n = F_{n-1} + F_{n-2}$ so $F_n - F_{n-1} - F_{n-2} = 0$. The auxiliary equation is $\lambda^2 - \lambda - 1 = 0$ Which has roots $\lambda = \frac{1 \pm \sqrt{5}}{2}$ Or $\phi$ and $-(\frac{1}{\phi})$ (the golden ratio phi). Hence $F_n = A(\frac{1 + \sqrt{5}}{2}) + B(\frac{1 - \sqrt{5}}{2}).$ We know $F_0 = 0, F_1 = 1$ so if we substitute these values in, we get that $A = \frac{1}{\sqrt{5}}, B = -\frac{1}{\sqrt{5}}$.

Actually by some magic we can further simplify down to $F_n = \left \lfloor{\frac{\phi^n}{\sqrt{5}}}\right \rfloor$

Non-homogenous recurrences have the $f(n)$ bit not 0.

Non-homogenous recurrences

Finding solutions to non-homogenous recurrences of the form $x_n + ax_{n-1} + bx_{n-2} = f(n)$

1. Find the general solution $x_n = h_n$ of the homogenous recurrence: $x_n + ax_{n-1} + bx_{n-2} = 0.$ (will have 2 arbitrary constants)
2. Find any particular solution $x_n = p_n$ of the original recurrence.
3. The general solution will be $x_n = h_n + p_n$.

Finding the particular solution $p_n$ is not straight forward, but the technique is to try solutions that are similar in form to $f(n)$. Substitute these into the original recurrence (as they are solutions!) and try solve.

If $f(n)$ is a constant, try find a constant $k$.

If $f(n)$ is a polynomial, try find a same degree polynomial. (substitute in a general polynomial)

Etc. just like differential equations.

Decimal Representation of Reals

Link to the PDF.

Rational numbers can be represented by expansions, so can repeating decimals.

Repeating decimals can be expressed as a fraction by summing an appropriate geometric series.

For example, $0.4\overline{9}$ can be expanded as follows;

The decimal expansion of any arbitrary real can be computed by squeezing between two converging sequences.

Expand if you really want to see...

If that real $x$ is not an integer, it is between one integer and the next consecutive one. Now divide the integer gap into 10 fractions - if $x$ is not on a fraction then it is between two consecutive division points, $a_0 + \frac{a_1}{10} < x < a_0 + \frac{a_1 + 1}{10}$ Where $a_1 \in \{0..9\}$. We can then repeat to get $a_0 + \frac{a_1}{10} + \frac{a_2}{10^2} < x < a_0 + \frac{a_1}{10} + \frac{a_2 + 1}{10^2}$ etc etc.

Every rational has a terminating or repeating decimal, there's some formal rigorous waffle and then you compute it via long division really.

Not sure why this is here save the fact that yes decimals are related to series.