16,278 reputation
32957
bio website mai.liu.se/~halun
location Linköping, Sweden
age 44
visits member for 3 years, 11 months
seen 17 hours ago

I liked this site better in the good old days when the questions were fewer and more interesting... I still drop in from time to time, though.


Jul
5
comment Time needed to algebraically solve system of $15$ nonlinear equations with parameters
Do you have any reason to expect that there is a closed-form solution? That's quite exceptional for nonlinear ODEs.
Jul
5
comment Fourier transform help for solving $u_t+u_{xxxx}+u_{xx}=0$
In the integrating factor, you should have $dt$ instead of $d\omega$.
Jul
5
comment Find the power series of $x\ln(1-x)$.
@EricTowers: What do you mean by that? Can you give an example? As long as the terms are interleaved in the right order, I see no problem, since each partial sum of the interleaved series will be the sum of partial sums from the two original series.
Jul
4
comment Solve $\lim_{x\to 0} \frac{\sin x-x}{x^3}$
Related: math.stackexchange.com/questions/157903/…
Jul
3
comment Total Integral of ordered joint probability is 1 ???
OK. (But it's not an area...)
Jul
3
comment Total Integral of ordered joint probability is 1 ???
What do you mean by "area under joint pdf"?? The integral?
Jul
3
comment Substituting for Taylor series
@David: It simply means $-r<x<r$.
Jul
2
comment Substituting for Taylor series
Perhaps of interest here: math.wpi.edu/Course_Materials/MA1023C00/tayseries/node1.html (especially "Theorem 358"(!)).
Jul
1
comment Laplacian of a scalar in tensor
en.wikipedia.org/wiki/Laplace%E2%80%93Beltrami_operator
Jun
30
comment Failure of differential notation
possible duplicate of Challenge: Demonstrate a Contradiction in Leibniz' differential notation
Jun
29
answered Weird derivatives implication
Jun
28
comment Special conformal killing fields - solving for integral curves.
To solve the algebraic equation $x/x^2=A$, note that it implies that $x=\lambda A$ for some scalar $\lambda$; thus $\lambda A/(\lambda^2 A^2)=A$, so $\lambda = 1/A^2$. Hence $x=A/A^2$.
Jun
28
comment Integrate $(x^n)\sin x$, where $n$ is an arbitrary natural number
This answer might be useful: math.stackexchange.com/questions/70974/…
Jun
28
comment Invariant Line of a System of Equations
@usainlightning: Not for any value of $k$! (Only if it satisfies $k=g(k)$.)
Jun
28
comment Why is any continous function integrable?
proofwiki.org/wiki/Continuous_Function_is_Riemann_Integrable
Jun
27
comment Is there an ambiguous case for cosine law. Why or why not.
@user157151: englishplus.com/grammar/00000175.htm
Jun
26
comment Function such that zeros$=$order of the derivative
I would guess that there is some orthogonality which explains why these polynomials have only real zeros.
Jun
26
comment Function such that zeros$=$order of the derivative
@cnick: It's easier than one would expect! See my answer below. ;-)
Jun
26
answered Function such that zeros$=$order of the derivative
Jun
26
comment Function such that zeros$=$order of the derivative
@Balla: $f(x)=x^n$ doesn't work. Then $f^{(n)}$ has no zeros (instead of $n$), etc.