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  1. W

    A Formula for this curve?

    The action potential satisfy a 2nd order ODE according to the Hodgkin-Huxley model. See equation (30) of their seminal paper [1]. This equation has no closed form solution, but can be solved numerically. [1]. HODGKIN AL, HUXLEY AF. A quantitative description of membrane current and its...
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    I Good introductory book for chaos theory?

    Nonlinear dynamics and chaos by Steven Strogatz is a classic and often the book used in introductory courses on the subject. It's exceptional well written and easily digestible. More advanced treatments of the subject depends on the direction you want to go in. Chaos is a big field with a lot of...
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    First order differential equation involving a square root

    I would expect the equation to be $$ \bigg(\frac{dR}{dt}\bigg)^2 = \frac{GM}{R} $$ if it where to describe the gravitational collapse of a non-relativistic star, as a fluid parcel located at distance ##R## is gravitational bound (the Viral theorem). But I might be wrong, I haven't really studied...
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    Solution of a parametric differential equation

    No worries! It was me that read your original post in a hurry. Your differential equation in ##y^\prime## belong to a notorious difficult class of ODE's called Abel's nonlinear ODE's of the fist-kind. I haven't had the change nor time to study this class of ODE's, so I'm afraid that I can't...
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    Solution of a parametric differential equation

    Your ODE is a second-order linear equation with constant coefficients. It is rather straightforward to solve, simply observe that you can write it in the following form $$ \big(e^{kt}y^\prime\big)^\prime = - ae^{kt}.$$ Now you simply have to integrate twice.
  6. W

    Solve this functional equation

    Let ##c## be a rational number (possibly irrational), then there exist a sequence ##\{q_n\}_{n\in\mathbb{N}}## of purely rational numbers that converges to ##c##. Therefore (assuming ##f## is continuous) $$ \begin{align*} f(cx) &= \lim_{n\rightarrow\infty}f(q_nx) \\ &=...
  7. W

    Solve this functional equation

    The function ##f(x) = \mathrm{max}(x,0)## (i.e. the positive part) is also a solution to Cauchy as well and though it is continuous it isn't differentiable at ##x=0## and thus not analytic. Check for yourself if also satisfy the original functional equation.
  8. W

    Solve this functional equation

    I believe that ##f(x) = ax## for ##a\in\{0,1\}## are the entire family of continuous solutions (consisting of elementary functions) to the original functional equation. As any continuous solution also has to be a solution to the Cauchy's functional equation. However there exist non-continuous...
  9. W

    Solve this functional equation

    True, however that just imply that either ##a=1## or ##a=0##.
  10. W

    Solve this functional equation

    From ##f(x^2)=xf(x)## conclude that ##f## is an odd function i.e. ##f(-x)=-f(x)##. If we take ##(x,y) = (-x,y)## then we get that $$ f(-xf(y)+f(-x)) + yf(y) = f(-x) + yf(y-x) $$ or, by using the odd property of ##f##, that $$ -f(xf(y)+f(x)) + yf(y) = -f(x) + yf(y-x). $$ Now, add this equation...
  11. W

    Solve this functional equation

    No, you are right. What I wrote in 19 wasn't correct.
  12. W

    Solve this functional equation

    That is also tremendous progress. This allow you to simplify the functional equation to $$xf(y) + yf(y) = yf(x+y)$$
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    Find a function satisfying these conditions: f(x)f(f(x)) = 1 and f(2020) = 2019

    Sure! However call ##f(x) = 2020##. Then, by an identical argument, we instead get that ##f(2020) = 1/2020##, which clearly contradicts the condition ##f(2020) = 2019##. I agree. If the function exist then its graph is intersting. It clearly isn't monotone.
  14. W

    Find a function satisfying these conditions: f(x)f(f(x)) = 1 and f(2020) = 2019

    Not sure if this is going to be of any help. But doesn't the condition ##f(2020)=2019## seem to be contradictory? I mean, let ##y=f(x)## for some arbitrary chosen ##x\in\mathbb{R}##. Then $$yf(y) = 1,$$ according to the functional equation. Or equivalently that $$ f(x) = \frac{1}{x}.$$ This...
  15. W

    Advection Diffusion problem

    Your PDE is linear, you should therefore try to look for a solution by separation of variables; i.e. assume that the solution is of the form ##T(t,x) = u(t)v(x)## and derive a space-independent equation for ##u(t)## and a time-independent equation for ##v(x)##. Remember that ##v(x)## inherit...
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    Orthogonality Relationship for Legendre Polynomials

    Legendre polynomials are orthogonal but not orthonormal over the interval ##[-1,1]##. Thus, you shouldn't expect your orthonormal basis to be identical to the Legendre polynomials. NB. If you are trying to construct and orthonormal set ##\{p_0,p_1,p_2\}## of polynomials over the interval...
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    B How to prove this infinite series?

    Your series can be put on the form of a telescope series and thereby summed up. $$ \begin{align*} \sum_{n=1}^\infty\frac{n^2+3n+1}{n^4+2n^3+n^2} &= \sum_{n=1}^\infty\frac{n^2+3n+1}{n^2(n+1)^2} \\ &= \lim_{N\rightarrow\infty}\sum_{n=1}^N\bigg\lbrace\frac{2n^2+3n}{(n+1)^2} -...
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    I A simple equation with simple solution - how to solve it?

    I agree. Therefore, let ##x = \sin\theta## and ##y = \sin\phi##, then the equation $$y-x=\sqrt{1-x^2} + \sqrt{1-y^2}$$ becomes $$ \begin{align*} \sin\phi - \sin\theta &= \sqrt{1-\sin^2\theta} + \sqrt{1-\sin^2\phi} \\ &= \cos\theta + \cos\phi \end{align*} $$ or (by the sum-to-product trig...
  19. W

    Showing that a function is surjective onto a set

    I assume (since you are not precisely specifying it) that ##B(0,r)\subseteq\mathbb{R}^n## is the closed ball of radius ##r>0## centered at ##0##. You are given that ##0\in B(0,1)## is a fixed point of ##f##. This immediately tells us that there exist a point in ##x\in B(0,1)## for which ##f(x)...
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    I How can you prove the integral without knowing the derivative?

    Here is an example of how you would go about computing a definite integral of ##f(x) = x^2## from "first-principle" without utilizing the fundamental theorem of calculus. I will only show the calculations for the right evaluated Riemann sum, as the calculations for the left evaluated Riemann sum...
  21. W

    Another Second Order ODE Problem (ML Boas, Ch 8, Sec 7, Prob 5)

    There are two tricks to solve this ODE. The first is to notice that $$(y^{\prime\prime})^2 = k^2\big[1+(y^\prime)^2\big]$$ is nothing more than a 1st-order ODE in ##y^\prime##. Thus, let ##p = y^\prime## (and thus ##p^\prime = y^{\prime\prime}##) and the original ODE becomes $$(p^\prime)^2 =...
  22. W

    Help solving this equation please: y^2-2ln(y)=x^2

    It is possible to express ##y## in terms of ##x##. From your calculations you have that $$y^{-2}e^{y^2} = e^{x^2},$$ this it great. Rearrange the equation to look like $$-y^2e^{-y^2} = -e^{-x^2}.$$ This is an equation of the form ##we^w = z## and can thus be solved using the Lambert ##W##...
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    I Solving an ODE with Legendre Polynomials

    Begin by deviding both side of $$\frac{d}{d\theta}\bigg(\sin\theta\frac{d\Theta}{d\theta}\bigg) = - l(l+1)\sin\theta\,\Theta$$ by ##\sin\theta## to get $$\frac{1}{\sin\theta}\frac{d}{d\theta}\bigg(\sin\theta\frac{d\Theta}{d\theta}\bigg) + l(l+1)\Theta = 0.$$ Now, introduce the new variable...
  24. W

    Chemistry Quantum Chemistry - Particle in a box

    Apply l'Hospital's rule to deduce that \begin{align*} \lim_{m\rightarrow\infty}\frac{m\pi \pm 2}{4m\pi} &= \frac{1}{4} \end{align*} and conclude that the probability for finding the particle in the leftmost quarter of the box is ##1/4## at high energies (i.e. large ##m##).
  25. W

    I Assigning a value for integrating a divergent oscillatory function to infinity

    Sure! However, as you probably know, an antiderivative to ##f(x) = x\sin(x)## is ##F(x) = \sin(x) - x\cos(x)##. Thus, the original improper integral is \begin{align} I &= \int_0^\infty x\sin(x)\,dx \\ &= \lim_{a\rightarrow\infty}\Big[\sin(x) - x\cos(x)\big]. \end{align} This limit doesn't exist...
  26. W

    Perturbation theory for solving a second-order ODE

    How sure are you that the equation reads $$\ddot{\xi} = -b\xi + \cos(\omega t)(a-c\xi^2)$$ and not instead $$\dot{\xi} = -b\xi + \cos(\omega t)(a-c\xi^2).$$ I only ask because the latter is an Ricatti equation and thus exactly solvable. If you only have calculated the pertubation series upto...
  27. W

    Why is my simulation of projectile trajectory with air resistance wrong?

    Congratulation! So what is the ideal angle when you include air resistance?
  28. W

    I Does this ODE have any real solutions?

    There are an infinite number of trivial solutions to this ODE. Supose ##f:\Omega\rightarrow\mathbb{C}## is a holomorphic function, then ##\big(y(x), z(x)\big) = \big(f(x),\pm f(x)\big)## are trivial solutions to your original ODE $$(y^\prime)^2 - (z^\prime)^2 + 2m^2(y^2 - z^2) = 0.$$ So to...
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