# Untitled Page 31

- Page ID
- 148375

## 7.4 Taylor series

If you calculate *e*^{0.1} on your calculator, you'll find that
it's very close to 1.1. This is because the tangent line at *x*=0
on the graph of *e*^{x} has a slope of 1 (*de*^{x}/*dx*=*e*^{x}=1 at *x*=0),
and the tangent line is a good approximation to the exponential curve
as long as we don't get too far away from the point of tangency.

How big is the error? The actual value of *e*^{0.1} is 1.10517091807565…, which
differs from 1.1 by about 0.005. If we go farther from the point of tangency,
the approximation gets worse. At *x*=0.2, the error is about 0.021, which is about
four times bigger. In other words, doubling *x* seems to roughly quadruple the error,
so the error is proportional to *x*^{2}; it seems to be about *x*^{2}/2. Well, if we want a handy-dandy, super-accurate
estimate of *e*^{x} for small values of *x*, why not just account for this
error. Our new and improved estimate is

for small values of *x*.

Figure b shows that the approximation is now extremely good for sufficiently small values of *x*.
The difference is that whereas 1+*x* matched both the y-intercept and the slope of the curve, 1+*x*+*x*^{2}/2 matches
the curvature as well. Recall that the second derivative is a measure of curvature. The second derivatives of the
function and its approximation are

We can do even better. Suppose we want to match the third derivatives. All the derivatives of *e*^{x}, evaluated
at *x*=0, are 1, so we just need to add on a term proportional to *x*^{3} whose third derivative is one. Taking the
first derivative will bring down a factor of 3 in front, and taking and the second derivative will give a 2, so
to cancel these out we need the third-order term to be (1/2)(1/3):

Figure c shows the result. For a significant range of *x* values close to zero, the approximation is
now so good that we can't even see the difference between the two functions on the graph.

On the other hand, figure d shows that
the cubic approximation for somewhat larger negative and positive values of *x* is poor --- worse, in fact, than the
linear approximation, or even the constant approximation *e*^{x}=1. This is to be expected, because
any polynomial will blow up to either positive or negative infinity as *x* approaches negative infinity, whereas
the function *e*^{x} is supposed to get very close to zero for large negative *x*. The idea here is that derivatives
are *local* things: they only measure the properties of a function very close to the point at which they're
evaluated, and they don't necessarily tell us anything about points far away.

It's a remarkable fact, then, that by taking enough terms in a polynomial approximation, we can always get as good an
approximation to *e*^{x} as necessary --- it's just that a large number of terms may be required for large values of *x*.
In other words, the *infinite series*

always gives exactly *e*^{x}. But what is the pattern here that would allows us to figure out, say, the fourth-order
and fifth-order terms that were swept under the rug with the symbol “...”? Let's do the fifth-order term as
an example. The point of adding in a fifth-order term is to make the fifth derivative of the approximation equal
to the fifth derivative of *e*^{x}, which is 1. The first, second, ... derivatives of *x*^{5} are

The notation for a product like 1⋅2⋅…⋅ *n* is n!, read “*n* factorial.” So to get a term
for our polynomial whose fifth derivative is 1, we need x^{5}/5!. The result for the infinite series is

where the special case of 0!=1 is assumed.^{2} This infinite series is called the *Taylor series* for *e*^{x}, evaluated around *x*=0, and it's true, although I haven't proved it,
that this particular Taylor series always converges to *e*^{x}, no matter how far *x* is from zero.

In general, the Taylor series
around *x*=0 for a function *y* is

where the condition for equality of the nth order derivative is

Here the notation .|_{x=0} means that the derivative is to be evaluated at *x*=0.

A Taylor series can be used to approximate other functions besides *e*^{x}, and when you ask your calculator to evaluate a
function such as a sine or a cosine, it may actually be using a Taylor series to do it.
Taylor series are also the method Inf uses to calculate most expressions involving infinitesimals. In example
13 on page 29, we saw that
when Inf was asked to calculate 1/(1-*d*), where *d* was infinitesimal, the result was the geometric series:

: 1/(1-d) 1+d+d^2+d^3+d^4

These are also the the first five terms of the Taylor series for the function *y*=1/(1-*x*), evaluated around *x*=0.
That is, the geometric series 1+*x*+*x*^{2}+*x*^{3}+… is really just one special example of a Taylor series, as
demonstrated in the following example.

### Example 5

◊ Find the Taylor series of*y*=1/(1-

*x*) around

*x*=0.

◊ Rewriting the function as *y*=(1-*x*)^{-1} and applying the chain rule, we have

The pattern is that the nth derivative is n!. The Taylor series therefore has a_{n}=n!/n!=1:

If you flip back to page 106 and compare the rate of convergence of the geometric
series for *x*=0.1 and 0.5, you'll see that the sum converged
much more quickly for *x*=0.1 than for *x*=0.5. In general, we expect
that any Taylor series will converge more quickly when *x* is smaller. Now consider what happens at *x*=1. The series
is now 1+1+1+…, which gives an infinite result, and we shouldn't have expected any better behavior, since
attempting to evaluate 1/(1-*x*) at *x*=1 gives division by zero. For *x*>1, the results become nonsense.
For example, 1/(1-2)=-1, which is finite, but the geometric series gives 1+2+4+…, which is infinite.

In general, every function's Taylor series around *x*=0 converges for all values of *x* in the range
defined by |*x*|<r, where *r* is some number, known as the radius of convergence.
Also, if the function is defined by putting together other functions that are well behaved (in the sense of
converging to their own Taylor series in the relevant region), then the Taylor series will not only converge
but converge to the *correct* value.
For the function *e*^{x}, the radius
happen to be infinite, whereas for 1/(1-*x*) it equals 1. The following example shows a worst-case
scenario.

### Example 6

The function *y*=*e*^{-1/x2}, shown in figure e, never converges to its Taylor series, except at *x*=0.
This is because the Taylor series for this function, evaluated around *x*=0 is exactly zero! At *x*=0, we have
*y*=0, *dy*/*dx*=0, d^{2} *y*/*dx*^{2}=0, and so on for every derivative. The zero function matches the function
*y*(*x*) and all its derivatives to all orders, and yet is useless as an approximation to *y*(*x*). The radius of convergence
of the Taylor series is infinite, but it doesn't give correct results except at *x*=0. The reason for this is that
*y* was built by composing two functions, *w*(*x*)=-1/*x*^{2} and *y*(*w*)=*e*^{w}. The function *w* is badly behaved at *x*=0
because it blows up there. In particular, it doesn't have a well-defined Taylor series at *x*=0.

### Example 7

◊ Find the Taylor series of*y*=sin

*x*, evaluated around

*x*=0.

◊ The first few derivatives are

We can see that there will be a cycle of sin, cos, -sin, and -cos, repeating
indefinitely. Evaluating these derivatives at *x*=0, we have
0, 1, 0, -1, .... All the even-order terms of the series are zero, and all the odd-order
terms are ±1/n!. The result is

The linear term is the familiar small-angle approximation sin *x*≈ *x*.

The radius of convergence of this series turns out to be infinite. Intuitively the reason for
this is that the factorials grow extremely rapidly, so that the successive terms in the series eventually start
diminish quickly, even for large values of *x*.

### Example 8

Suppose that we want to evaluate a limit of the form where*u*(0)=

*v*(0)=0. L'H\^{o}pital's rule tells us that we can do this by taking derivatives on the top and bottom to form

*u*'/

*v*', and that, if necessary, we can do more than one derivative, e.g.,

*u*”/

*v*”. This was proved on p. 152 using the mean value theorem. But if

*u*and

*v*are both functions that converge to their Taylor series, then it is much easier to see why this works. For example, suppose that their Taylor series both have vanishing constant and linear terms, so that

*u*=

*ax*

^{2}+… and

*v*=

*bx*

^{2}+…. Then

*u*”=2

*a*+…, and

*v*”=2

*b*+….

A function's Taylor series doesn't have to be evaluated around *x*=0. The Taylor series around some other
center *x*=*c* is given by

where

To see that this is the right generalization, we can do a change of variable,
defining a new function *g*(*x*)=*f*(*x*-*c*). The radius of convergence is to be measured
from the center *c* rather than from 0.

### Example 9

◊
Find the Taylor series of ln *x*, evaluated around *x*=1.

◊ Evaluating a few derivatives, we get

Note that evaluating these at *x*=0 wouldn't have worked, since division by zero is undefined;
this is because ln *x* blows up to negative infinity at *x*=0. Evaluating them at *x*=1, we
find that the *nth* derivative equals ± (n-1)!, so the coefficients of the Taylor
series are ± (n-1)!/n!=±1/n, except for the *n*=0 term, which is zero because
ln 1=0. The resulting series is

We can predict that its radius of convergence can't be any greater than 1, because
ln *x* blows up at 0, which is at a distance of 1 from 1.