# Recent content by matteo86bo

1. ### How to solve an integral with the Inverse error function

Thanks, the problem seems very complicated and I think I have to resort to a numerical integration. However, that would be easier if one could write \mathcal{I}(\xi,\beta)=f(\beta)\mathcal{I}^{\prime}(\xi) so this way I would have to integrate only once. Do you see a way to possibly achieve that?
2. ### How to solve an integral with the Inverse error function

Thank you for your answer. But I don't understand how I can Taylor expand where the integrand diverges. Also, do you see a way to normalize the integral and/or make it more simple to solve it numerically. What I find challenging is that for every value of beta I have to numerically solve the...
3. ### How to solve an integral with the Inverse error function

Hi, this is not a homework and my problem is much bigger for me to give full details here. I came across this integral \mathcal{I}(\xi)=\int^{\xi_c}_{\xi}{\rm d}\xi^\prime\exp\left[\sqrt{2}\sigma\,{\rm Erf}^{-1}\left(1-\frac{8\pi}{3}{\xi^\prime}^3\right)\right] where Erf^{-1} is the...
4. ### I'm stuck in grad school

I won a scolarship two years ago but my supervisor is not really paying for me. But you're right, I have to talk to him as soon as possible. I'm just very afraid of doing it because I think I might lose it and compromise my future. I'm really pissed off because I won the scholarship because I...
5. ### I'm stuck in grad school

It sucks because I only have one year left and I really don't know where else to turn. I do know how to program but I certainly don't have the skills to work in a big company that require senior positions. Do you know where I can look for companies that hire programmers and also offer some sort...
6. ### I'm stuck in grad school

I'm located in Europe and the PhD only last three years. I can program in Python, Fortran and C and I am bilingual. My supervisor keeps saying that we have to perfect things and he's afraid of comments of possible referees, that's why I'm not publishing anything.
7. ### I'm stuck in grad school

I have a MSc in astrophysics and I'm a last year PhD student. My project is not going anywhere, I haven't published any paper and I'm far from finishing. My advisor is very picky and won't let me publish anything and I'm terrified the last years have been a huge waste of time. What do you think...
8. ### Numerical stability of stiff problem

I have a very complicated stiff system, which can be expressed as: x^{\prime}(t)=A(x,y)-B(x,y) y^{\prime}(t)=C(x,y)-D(x,y) I decided to solve it with the fixed-point iteration method (http://en.wikipedia.org/wiki/Fixed_point_iteration) but I also have to use adaptive time stepping to...
9. ### Discretization of the divergence operator

I work with a grid-based code, this means that all of my quantities are defined on a mesh. I need to compute, for every point of the mesh the divergence of the velocity field. All I have is, for every cell of my mesh, the values of the 3-d velocity in his 26 neighbors. I call neighbors the...
10. ### Difficult non-linear differential equation

Ok, now I got it, thanks a lot for you time!
11. ### Difficult non-linear differential equation

Sorry but I don't understand how to compute the Ks considering them as arrays. I tried googling it but I couldn't find anything. Can please you make an example?
12. ### Difficult non-linear differential equation

Ah ok, thanks. So in my case, since I don't have an explicit time dependency the ks should be k1=hf(x_{i},v_{i}) k2=hf(x_{i}+k1/2,v_{i}+k1/2,) k3=hf(x_{i}+k2/2,v_{i}+k2/2) k4=hfx_{i}+k3,v_{i}+k3) Is that right?
13. ### Difficult non-linear differential equation

Thanks that helped but I have still some problem in computing v. According to what you wrote and the runge Kutta IV method definition the k should be: k1=hf(t_{i},x_{i}) k2=hf(t_{i+1/2},x_{i}+k1/2) k3=hf(t_{i+1/2},x_{i}+k2/2) k4=hf(t_{i+1},x_{i}+k3) How is v for every k?
14. ### Difficult non-linear differential equation

Homework Statement I need to solve this differential equation numerically. I hope you can help me, it's not homework! Homework Equations This is the equation to be solved x''=\dfrac{\alpha}{x}-\beta\dfrac{(x'-\gamma x)^2}{x}-\delta x - \dfrac{\varepsilon}{x^2} The Attempt at a...
15. ### How to trasform an orthonormal system in two reference frames

My question is not homework. I feel ashamed of having this doubts but I'm really stuck on this. The problem is I have a reference frame xyz and here I define the COM \vec x{_{cm}} of the system. Now I move the COM reference frame x'y'z': \vec{x'}=\vec{x}-\vec x{_{cm}} In this reference frame I...