Sequences

1. So we tried dealing with infinite series as if they were polynomials. This caused problems.

One solution was to consider the sequence of partial sums, and treat the series converging to whatever the sequence converges to.

But we have a new problem: what’s the criteria for a sequence converging?

2. We define a “Sequence” denoted \{a_{n}\} is a function whose domain is the set of all positive integers and whose range is a subset of the real numbers \mathbb{R}.

Example 1. Consider a_{n}=n^{-3}. We see as n\to\infty we have a_{n}\to 0.

We write this as \displaystyle\lim_{n\to\infty}\frac{1}{n^{3}}=0.

Example 2. Consider a_{n}=\cos(n\pi/2). So a_{2k-1}=0 for k=1,2,3,\dots and a_{2k}=(-1)^{k}.

The sequence doesn’t settle down, it’s always oscillating. So the limit does not exist.

3. A sequence \{a_{n}\} “Converges” if there is a finite number L such that \displaystyle \lim_{n\to\infty}a_{n}=L.

A sequence “Diverges” if \displaystyle \lim_{n\to\infty}a_{n}=\pm\infty or if the limit does not exist.

A sequence either converges or diverges. There is no middle ground, and there is no sequence that neither diverges nor converges. It must do one or the other.

Example 3. Consider the sequence
(1)\displaystyle a_{n}=\frac{3n^{3}+2n+1}{7n^{3}+4}
Does it converge or diverge? Well, one trick we can do is divide the top and bottom by n^{3}, giving us
(2)\displaystyle   \begin{aligned}a_{n}&=\frac{3n^{3}+2n+1}{7n^{3}+4}\left(\frac{1/n^{3}}{1/n^{3}}\right)\\  &=\frac{3+2n^{-2}+n^{-3}}{7+4n^{-3}}\\  &=\frac{3+\mathcal{O}(n^{-1})}{7+\mathcal{O}(n^{-1})}  \end{aligned}
Taking the limit as n\to\infty becomes easier, since it amounts to setting the \mathcal{O}(n^{-1}) terms to zero. That is:
(3)\displaystyle   \lim_{n\to\infty}\frac{3+\mathcal{O}(n^{-1})}{7+\mathcal{O}(n^{-1})}  =\frac{3+0}{7+0}=\frac{3}{7}.
Moreover, the sequence converges.

Example 4. Consider the sequence
(5)\displaystyle a_{n}=\frac{2n^{4}+5n+8}{n^{2}+1}
We divide the top and bottom by n^{2}
(6)\displaystyle \frac{2n^{4}+5n+8}{n^{2}+1}\left(\frac{n^{-2}}{n^{-2}}\right)=\frac{2n^{2}+5n^{-1}+8n^{-2}}{1+n^{-2}}
Again we write this using big O notation,
(7)\displaystyle \frac{2n^{2}+5n^{-1}+8n^{-2}}{1+n^{-2}}=\frac{2n^{2}+\mathcal{O}(n^{-1})}{1+\mathcal{O}(n^{-1})}
Taking the limit as n\to\infty produces a bit of a problem, because the sequence behaves like 2n^{2}. How can we say this? Because the bonus parts grouped into the \mathcal{O}(n^{-1}) terms are effectively thrown away (they vanish when we take the limit). Whatever is left over tells us the sequence’s behaviour.

So we see that
(8)\displaystyle \lim_{n\to\infty}\frac{2n^{2}+\mathcal{O}(n^{-1})}{1+\mathcal{O}(n^{-1})}=+\infty
which implies the sequence from Equation (5) diverges.

4. Formal Criteria, Useful Theorems. We can be more precise. Given a sequence \{a_{n}\} we have \displaystyle\lim_{n\to\infty}a_{n}=L if and only if for any \varepsilon>0 there exists a positive integer N such that n>N implies |a_{n}-L|<\varepsilon.

Theorem 1. If -1<r<1, then $\lim_{n\to\infty}r^{n}=0$.

Sandwich Theorem. If a_{n}\leq b_{n}\leq c_{n} for each n=1,2,3,\dots, and
\displaystyle \lim_{n\to\infty}a_{n}=\lim_{n\to\infty}c_{n}=L
then \displaystyle\lim_{n\to\infty}b_{n}=L.

Theorem 2. If \lim_{n\to\infty}|a_{n}|=0, then \lim_{n\to\infty}a_{n}=0.

Remark. The proof is easy, since $-|a_{n}|\leq a_{n}\leq|a_{n}|$ for each n. Then apply the Sandwich theorem.

Theorem 3. For any constant k we have
\displaystyle \lim_{n\to\infty}\frac{k^{n}}{n!}=0.

And we always should remember L’Hopital’s rule when considering tricky limits.

Advertisements

About Alex Nelson

I like math. I like programming. Most of all, I love puzzles.
This entry was posted in Analysis, Sequences. Bookmark the permalink.

One Response to Sequences

  1. Pingback: Series Convergence: What Does It Mean? | My Math Blog

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s