1. Notes are from D. R. Wilton, Dept. of ECE
Fall 2016
David R. Jackson
Notes 12
Asymptotic Series
Notes are from D. R. Wilton, Dept. of ECE

2. Asymptotic Series An asymptotic series (as z  ) is of the form or
Note the “asymptotically equal to” sign.
The asymptotic series shows how the function behaves as z gets large in magnitude.
Important point:
An asymptotic series does not have to be a converging series.
(This is why we do not use an equal sign.)

3. Asymptotic Series (cont.)
Properties of an asymptotic series:
For a fixed number of terms in the series, the series get more accurate as the magnitude of z increases.
For a fixed value of z, the series does not necessarily get more accurate as the number of terms increases.
The series does not necessarily converge as we increase the number of terms, for a fixed value of z.

4. Big O and small o notation
This notation is helpful for defining and discussing asymptotic series.
Big O notation:
Qualitatively, this means that f “behaves like” g as z gets large.
Definition: There exists a constant k and a radius R such that
For all

5. Big O and small o notation (cont.)
Examples:

6. Big O and small o notation (cont.)
Qualitatively, this means that f “is smaller than” g as z gets large.
Definition: For any  there exists a radius R (which depends on ) such that
For all

7. Big O and small o notation (cont.)
Examples:

8. Definition of Asymptotic Series
In order for this to be an asymptotic series we require the following:
As z gets large, the error in stopping at term n = N is smaller than this last term.
Example:

9. Definition of Asymptotic Series
Theorem If
Then
Note: This theorem gives us a good way to estimate the error as z gets large!

10. Definition of Asymptotic Series
Proof of theorem
Assume
(from definition of asymptotic series)

11. Summing Asymptotic Series
One must be careful when summing an asymptotic series, since it may diverge: it is not clear what the optimum number of terms is, for a given value of z = z0.
General “rule of thumb”:
Pick N so that the Nth term in the series is the smallest.

12. Generation of Asymptotic Series
Various method can be used to generate an asymptotic series expansion of a function.
Asymptotic methods such as the method of steepest descent.
Integration by parts.
Watson’s Lemma.
Other specialized techniques.

13. Example The exponential integral function: Branch cut
Note: E1 (z) is discontinuous (by 2i) across the negative real axis.

14. Example (cont.)
Use integration by parts:

15. Example (cont.) Using integration by parts N times: or
Question: Is this a valid asymptotic series?
Note: a0 = 0 here.

16. Example (cont.)
Examine the difference term: or so

17. Example (cont.) Hence Question: Is this a converging series?
Use the d’Alembert ratio test:
The series diverges!

18. Example (cont.) n An 1
0.2 2
-0.04 3
0.016 4 5 6 7 8
-0.013 9
0.021 10
-0.037
n = odd
n = even
Exact value =
Using n = 5 or 6 is optimum for x = 5. This is where |An| is the smallest.

19. Example (cont.) As x gets large, the error in stopping with N terms
is approximately given by the first term that is omitted.
To see this, use:
(from definition of asymptotic series)
Therefore, we have
Hence

20. Example (cont.)
N = 1
N = 2

21. Note on Converging Series
Assume that a series converges for all z  0, so that
Then it must also be a valid asymptotic series:
This is a converging series that approaches a constant as z gets large. (It is a Taylor series if we let w = 1/z.)
Proof:

22. Note on Converging Series (cont.)
Example:
The point z = 0 is an isolated essential singularity,
and there are no other singularities out to infinity.
This Laurent series converges for all z  0.
This is a valid asymptotic series.