MAT334-2018F > Quiz-2

Q2 TUT 5201

(1/1)

**Victor Ivrii**:

Show that each of the following series converges for all $z$:

\begin{align*}

&\sum_{n=0}^\infty \frac{z^n}{n!}, && \sum_{n=0}^\infty (-1)^n \frac{z^{2n}}{(2n)!}.

\end{align*}

**Ge Shi**:

(1)

Apply ratio test:

|[Z^n+1 / (n+1)!] / [Z^n / n!]|=|Z| / n+1

Limit |Z| / n+1= 0 < 1 as n approaches infinity

thus it converges for all z

(2)

Apply ratio test:

|[(-1)^n+1*Z^2(n+1) / (2n+1)!]/ [(-1)^n*Z^2n / (2n)!]| = |Z^2| / 2n+1

Limit |Z^2| / 2n+1 = 0 < 1 as n approaches infinity.

thus it converges for all z

Beyond readability (and sanity)

**Jeffery Mcbride**:

Every power series has a convergence radius R, where Sum[anxn] converges if |x| < R.

The first summation is the power series equal to ez

and we have an = 1/n!

lim |an+1| / |an| = 1 / (n + 1)

= 0. So our convergence radius R is infinity and the power series converges for all z.

The second summation is the power series equal to cos(z) and we know:

cos(z) = (1/2)*(eiz + e-iz)

From the first series, we know the ez is convergent on all z, so cos(z) is also convergent on all z.

**hanyu Qi**:

In attachment.

**Victor Ivrii**:

Jeff, learn a bit of LaTeX, since without it anything but the most simple math expressions will be out of your reach

http://forum.math.toronto.edu/index.php?topic=610.0

Alex, learn how to scan properly

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