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Ordinary or partial differential equations. Delay differential equations, neutral equations, integro-differential equations. Well-posedness, asymptotic behavior, and related questions.
23
votes
Is there a general solution for the differential equation $f''(x) = f(f(x))$?
The equation has solutions with powers, $f(x) = ax^b$. Inserting this ansatz, one has
$$
a b (b-1) x^{b-2} = a (a x^b)^b = a^{b+1} x^{b^2} \ ,
$$
so the requirements on $a$ and $b$ are
$$
b-2 = b^2 \ …
17
votes
Motivation and physical interpretation of the Laplace transform
The Laplace transform is the fundamental operation encoding the canonical ensemble in statistical mechanics. It converts the density of states $d(\varepsilon )$ (a non-statistical concept) into the ca …
10
votes
Accepted
Taylor expansion of exponential of a Lie derivative
Maybe I'm missing something, but it seems to me you're overthinking this. Since
$$
\int_{0}^{1} d\theta (1-\theta ) \theta^{n} = \frac{1}{n+1} -\frac{1}{n+2} = \frac{n!}{(n+2)!}
$$
the third term in y …
3
votes
Global first integral for certain $3$ dimensional system
One can find a solution of the form $x=y=z$, namely, $x=2$arccot$(\exp (-t-a))$ with the free parameter $a$. Of course, there should be more. Note also the symmetries of the problem: For any solution …
2
votes
Accepted
Example of differential equation with periodic boundary conditions that has at least two sim...
For the choice $q(x)=2q\cos (2x)$ (with $q$ on the right hand side a constant, I'm trying to stick to standard notation), the differential equation is known as Mathieu's equation, with solutions descr …
2
votes
Hypergeometric function with changed argument
The relationship can be written using the dilation operator as
$$
_2 F_1 (a,b,c,p\cdot z) = \exp \left( \ln p \cdot z \frac{\partial}{\partial z} \right)\, {}_2 F_1 (a,b,c,z)
$$
1
vote
What is an "exact solution" to a PDE?
There is no "exact answer" to this question. Answers will contain the words "reasonable" and "appropriate", terms which depend on the context. I'll try to give a reasonable answer.
Given a domain of p …
1
vote
Accepted
How to force my differential equations give a bounded solution?
You could just modify your second equation once $t>24$, e.g.,
$$
B'(t) = \theta(24-t) [c (1 - B(t)) \theta(18 - t) - d B(t) S(t) - 3 c (1 - B(t - 18))]-\frac{1}{2}\theta(t-24)B(t)
$$
If a smoother mod …