# Method of successive approximation

• Tsunami
In summary, last semester, the Rayleigh-Schrödinger perturbation theory was discussed in the course QM 2, which is based on a general principle of equating terms with the same nth order dependence. This principle is used in thermoacoustics as well, but the name of this abstract theory is not known. It may be related to Fredholm or Volterra integral equations, but this is uncertain. The smallness of the parameter only helps in truncating the series, while the idea behind perturbation theory is to solve a family of problems parametrized by the small parameter. This leads to a polynomial equation in the parameter, which can only be solved if all coefficients are equal to zero, resulting in the pert

#### Tsunami

Last semester in my course QM 2 we discussed the Rayleigh-Schrödinger perturbation theory. A very elegant theory, based on a general principle: when terms are depending on a certain factor to the nth order, where the factor is very small if not infinitesimally small, you can collect the terms that have the same nth order dependence, and that state that:

the terms of the 0th order are the most prominent
the terms of the 1th order are second most prominent
etc.

(I'm a bit clumsy at wording this in English, my apologies.)

Now, this semester I'm working on a individual project about thermoacoustics, and I'm using a similar method of perturbation theory - the principle of order division is the same, only the context is different. I've been using this trick without ever questioning where it came from - however, it's time to write a report about it, so I feel I really should know where it came from. However, I can't seem to find a good site about this, and considering I don't even know the name of such a theory, it's not easy to look for it either.

So, my question is: what is the name of the abstract theory that describes this method of succesive approximation? It's always nice if you can provide a lucid link as well, but providing me a name would be a great help already. Thanks.

Tsunami said:
Last semester in my course QM 2 we discussed the Rayleigh-Schrödinger perturbation theory. A very elegant theory, based on a general principle: when terms are depending on a certain factor to the nth order, where the factor is very small if not infinitesimally small, you can collect the terms that have the same nth order dependence, and that state that:

the terms of the 0th order are the most prominent
the terms of the 1th order are second most prominent
etc.

I can't really help you with the name, sorry. It is really the core of perturbation theory, but I can't really put a name on it. You could look up Fredholm or Volterra integral equations, but I don't know if that's what you're looking for.

However, I'd like to point out something else. The fact that you can equate the different orders has nothing to do with your parameter being "small". The smallness of the parameter only comes into cut off the series.
But the idea behind the perturbation series is in fact different. Instead of solving ONE problem, you solve a WHOLE family of problems, parametrized in your small parameter (say, lambda). If you can now write out equations for that whole family of problems, then things should be ok, because the solution you're looking for is then found by putting in your "correct" value of lambda. But if you leave lambda "free" because you look for the whole family of solutions, and you arrive at an equation:

blabla + lambda . blah1 + lambda^2 . blah2 +... = blublu + lambda. bluh1 +...

then you are in fact solving a POLYNOMIAL equation in lambda, which looks trivial:

(blabla - blublu) + lambda . (blah1 - bluh1) + lambda^2 (blah2 - bluh2) + ... = 0.

Now, for a polynomial to be 0 FOR ALL VALUES OF LAMBDA, this can only happen when all of its coefficients, individually, are 0.

So from this follows that:
blahblah = blublu
blah1 = bluh1
...

which are nothing else but your perturbative equations of different order.

cheers,
Patrick.

The method of successive approximation, also known as perturbation theory, is a powerful tool in many areas of physics and mathematics. It allows us to solve complex problems by breaking them down into simpler parts that can be solved iteratively. This method is based on the idea that when dealing with a small or infinitesimal factor, we can collect terms with the same order of dependence and treat them as a whole, with the terms of the lowest order being the most prominent. This approach is commonly used in quantum mechanics, thermoacoustics, and many other fields.

The specific theory you are referring to in your question is called Rayleigh-Schrödinger perturbation theory. This theory is a mathematical framework that allows us to calculate the energy levels and wavefunctions of a quantum system that is slightly perturbed from a known system. It is a generalization of the perturbation theory developed by Lord Rayleigh and Erwin Schrödinger, hence the name.

- An article on the method of successive approximation by the University of Colorado Boulder: https://www.colorado.edu/physics/phys4810/phys4810_fa18/lecture_notes/lec22.pdf
- A lecture on Rayleigh-Schrödinger perturbation theory by MIT OpenCourseWare: https://ocw.mit.edu/courses/physics...-fall-2013/lecture-notes/MIT8_05F13_Lec22.pdf
- A video explaining the basics of perturbation theory by Khan Academy: https://www.khanacademy.org/science/physics/quantum-physics/quantum-structures/v/perturbation-theory
- A comprehensive book on perturbation theory by G. Barton titled "Elements of Green's Functions and Propagation: Potentials, Diffusion, and Waves": https://books.google.com/books/abou...d_Propag.html?id=Jz6qBgAAQBAJ&source=kp_cover

I hope this helps you in your research and understanding of this important method in physics. Best of luck with your project!

## What is the Method of Successive Approximation?

The Method of Successive Approximation is a numerical method used to obtain approximate solutions to mathematical problems. It involves repeatedly refining an initial estimate until a desired level of accuracy is achieved.

## How does the Method of Successive Approximation work?

The method involves starting with an initial guess or estimate for the solution and then using this estimate to calculate a new, more accurate estimate. This process is repeated until the desired level of accuracy is reached.

## What types of problems can be solved using the Method of Successive Approximation?

The Method of Successive Approximation can be used to solve a wide range of problems, including systems of linear equations, differential equations, and optimization problems.

## What are the advantages of using the Method of Successive Approximation?

One of the main advantages of this method is its simplicity. It is relatively easy to understand and implement, making it a popular choice for solving numerical problems. It also allows for a high level of control over the accuracy of the solution.

## What are the limitations of the Method of Successive Approximation?

The main limitation of this method is that it can be time-consuming and computationally intensive, especially when solving complex problems. Additionally, the method may not always converge to the exact solution, only an approximation.