- #1
falbani
- 8
- 0
Hi. This is my first post in PF. I'm an undergraduate student of Electronics Engineering with strong interest in math & physics (and weak understanding of them :P).
One of the things I always hate is when in some book (or some lectures) ODEs or PDEs are solved after the magic words "[...] and by virtue of separation of variables [...]", and do not care to specify under which conditions such procedure is valid and guarantees uniqueness of solution.
For the sake of unambiguity, I'm talking about the technique that imply searching only for solutions of the form F(x, y) = X(x) · Y(y).
For example, take the Wave Equation, a 2nd order PDE:
[tex]\nabla^2 u - \frac{1}{c^2} \frac{\partial^2 u }{\partial t^2} = 0[/tex]
for which we know that a general solution is of the form:
(supose 1D for simplicity)
[tex] u(x, t) = g(x - ct) + h(x + ct) [/tex].
I can't see any way of expresing that as a product of a function exclusively of 'x' and a function exclusively of 't'. Where I would ended with 'separation of variables'?
Other questions:
How this technique doesn't restrict the solution? I'm always afraid of "losing" solutions on the way.
I have seen vague statements about the validity of this such us "time and space are independent" (maybe I am missquoting). Can anybody make sense of that? (Any conection with the fact that the joint density of independent random variables is the product of their densities?)
I searched on books and internet (this forum included) and not been able to satisfy all my doubts.
I'm after a deep understanding of the electromagnetic vector wave equation and I will be grateful if you can help me with this.
One of the things I always hate is when in some book (or some lectures) ODEs or PDEs are solved after the magic words "[...] and by virtue of separation of variables [...]", and do not care to specify under which conditions such procedure is valid and guarantees uniqueness of solution.
For the sake of unambiguity, I'm talking about the technique that imply searching only for solutions of the form F(x, y) = X(x) · Y(y).
For example, take the Wave Equation, a 2nd order PDE:
[tex]\nabla^2 u - \frac{1}{c^2} \frac{\partial^2 u }{\partial t^2} = 0[/tex]
for which we know that a general solution is of the form:
(supose 1D for simplicity)
[tex] u(x, t) = g(x - ct) + h(x + ct) [/tex].
I can't see any way of expresing that as a product of a function exclusively of 'x' and a function exclusively of 't'. Where I would ended with 'separation of variables'?
Other questions:
How this technique doesn't restrict the solution? I'm always afraid of "losing" solutions on the way.
I have seen vague statements about the validity of this such us "time and space are independent" (maybe I am missquoting). Can anybody make sense of that? (Any conection with the fact that the joint density of independent random variables is the product of their densities?)
I searched on books and internet (this forum included) and not been able to satisfy all my doubts.
I'm after a deep understanding of the electromagnetic vector wave equation and I will be grateful if you can help me with this.