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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 \ …
Michael Engelhardt's user avatar
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 …
Michael Engelhardt's user avatar
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 …
Michael Engelhardt's user avatar
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 …
Michael Engelhardt's user avatar
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 …
Michael Engelhardt's user avatar
2 votes

Hypergeometric function with changed argument

The relationship can be written using the dilation operator as $$ _2 F_1 (a,b,c,p\cdot z) = \exp \left( \ln p \cdot z \frac{\partial}{\partial z} \right)\, {}_2 F_1 (a,b,c,z) $$
Michael Engelhardt's user avatar
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 …
Michael Engelhardt's user avatar
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 …
Michael Engelhardt's user avatar