# Asymptotic Methods - Method of Steepest Descents

• Tangent87
In summary, the speaker is seeking help with applying the method of steepest descents to a specific example. They have correctly identified the saddle points and calculated the paths of steepest descent, but are struggling with deforming the current Hankel contour onto these paths. The expert suggests considering different paths for certain values of y and potentially working with t rather than x and y to simplify the calculations.
Tangent87
Hi, I'm hoping someone on here has some experience with using the method of steepest descents as I wasn't taught the course very well last term at university and I'm having trouble applying the method outlined in the books to my own examples. Specifically, I'm doing question B2/18 on page 73 here: http://www.maths.cam.ac.uk/undergrad/pastpapers/2001/Part_2/list_II.pdf

So far I've identified the saddle points of $$\phi(t)=\frac{t-t^{-1}}{2}$$ as $$t=\pm i$$ and then writing t=x+iy we have:

$$\phi(x+iy)=\left(\frac{x}{2}+\frac{x}{x^2+y^2}\right)+i\left(\frac{y}{2}-\frac{y}{x^2+y^2}\right)$$

The paths of steepest descent are given by $$Im\phi=\pm 1$$ which leads to the two paths being (I think):

$$x=-\sqrt{\frac{y(2+2y-y^2)}{y-2}}$$
(t=+i)

and $$x=-\sqrt{\frac{y(2-2y-y^2)}{y+2}}$$
(t=-i)

Where we take the negative roots so that $$Re\phi<0$$ which we require for steepest descent rather than ascent.

I'm now kind of stuck as these are pretty messy paths and even if I were to sketch them I'm not sure how to "deform" the current Hankel contour we have for the integral, onto them.

Last edited by a moderator:

Hi there,

I am a scientist with experience using the method of steepest descents. I understand that you are having trouble applying the method to your specific example and I am happy to help.

Firstly, I want to commend you on correctly identifying the saddle points of your function \phi(t). This is an important step in the method of steepest descents.

Next, I want to address your concern about the paths of steepest descent being messy and difficult to deform onto your current Hankel contour. This is a common issue when using this method and it is important to remember that the paths of steepest descent are not necessarily the actual paths that the integral will follow. They are simply a tool to help us evaluate the integral.

In your case, it looks like you have correctly calculated the paths of steepest descent as x=-\sqrt{\frac{y(2+2y-y^2)}{y-2}} and x=-\sqrt{\frac{y(2-2y-y^2)}{y+2}}. However, these paths are only valid for y\neq 2 and y\neq -2 respectively. This means that you will need to consider different paths for these values of y.

Additionally, it may be helpful to consider the paths in terms of t rather than x and y. This could potentially simplify your calculations and make it easier to deform the contour onto the paths of steepest descent.

I hope this helps and please feel free to reach out with any further questions. Good luck with your problem!

## 1. What is the Method of Steepest Descents?

The Method of Steepest Descents is a mathematical technique used to approximate the behavior of a function near a point of interest. It is often used to solve integrals and differential equations that involve complex functions.

## 2. How does the Method of Steepest Descents work?

The Method of Steepest Descents works by finding the path of steepest descent for a given function. This path is then used to approximate the behavior of the function near a point of interest, allowing for the evaluation of integrals and differential equations.

## 3. What are the applications of the Method of Steepest Descents?

The Method of Steepest Descents has a wide range of applications in various fields, including physics, engineering, and finance. It is often used to solve problems involving complex integrals and differential equations, as well as in optimization and approximation techniques.

## 4. What are the limitations of the Method of Steepest Descents?

The Method of Steepest Descents is not always accurate and can sometimes lead to incorrect results. It also requires a certain level of mathematical knowledge and may not be suitable for all problems. Additionally, it may not be suitable for functions with multiple critical points.

## 5. How does the Method of Steepest Descents compare to other asymptotic methods?

The Method of Steepest Descents is one of the most commonly used asymptotic methods, along with the Method of Stationary Phase and the Method of Saddle Point. Each method has its own strengths and limitations and is best suited for different types of functions and problems. It is important to carefully consider the properties of the function and the problem at hand when choosing which asymptotic method to use.

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