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Mar
15
comment A variational problem with a lagrangian , what is the lagrangian?
THANKS a lot could yuou help me with other problems i have about variatonal calculus :D i must prove 20 exercises but i have only solved 4 :D
Feb
20
comment minimal surface from a variational problem
i have donwloade the book and in example 8.1.2 they put only the Euler lagrange equation i put in my question they do not explicitly evaluate the mean curvature of the surface
Feb
20
comment minimal surface from a variational problem
of course but i know would need the curvature equation in $ R^{n} $ to see that the vanishing of Euler lagrange equation is the vanishing also from the mean curvature in n-dimension
Feb
20
comment minimal surface from a variational problem
io know i should compare my Euler lagrange equation with the differential equation of the mean curvature am i right ? where could i find more hitns ?
Feb
20
comment minimal surface from a variational problem
i found this problem on Evans partial differential equations chapter 8 books.google.es/books/about/… chapter 8 problem 12 :)
Feb
18
comment A variational problem with a lagrangian , what is the lagrangian?
shouldn't this be the term $ e^{f(x)}$ instead ?= in both cases ??
Feb
18
comment A variational problem with a lagrangian , what is the lagrangian?
thank you RObert :D wehn i have enoguht poitn to vote i will give you a point thanks again
Feb
18
comment Solve $y'(t) = \dfrac{1}{1+ty}$
wolframalpha.com/input/…
Feb
18
comment Solve $y'(t) = \dfrac{1}{1+ty}$
i suggest to define a new function $ u(t)=ty(t) $ to see what happens
Feb
13
comment Find solution for the following system: $x'=y^3-4x$ , $y'=y^3-y-3x$
you know $ y-x=f(t) $ for some function now you should replace this identity inside your equation :D
Feb
13
comment Find solution for the following system: $x'=y^3-4x$ , $y'=y^3-y-3x$
what do you jean by 'invariant' xy is time invariant so $ \frac{d (xy)}{dt }=0 $
Feb
8
comment Can I express $\sqrt{x}$ as $Q(x)$ where Q(x) is a first degree polynomial?
you can use the identity $ (\sqrt x +1)(\sqrt x -1)=x-1 $ to approximate the root by a rational function
Feb
5
comment The Laurent series of the digamma function at the negative integers
but i do not get it if we put $\Psi (x)= \frac{1}{z}-\gamma + \sum_{n=1}^{\infty}\zeta (k+1)(-z)^{k} $ how do you get then that $ \psi (1)= -\gamma $ this is my doubt :D thanks for your answer
Feb
3
comment Minus sign differential operators
the minus sign is to make the operator selfadjoint and so all the eigenvalues are always real and positive as a counter example imagine $ \frac{d^{2}f}{dx}= \lambda_{n}f(x) $ with conditions $ y(0)=y(1)=0$ are the eigenvalues real
Feb
1
comment Weak derivative of one parameter group and the domain of its generator
your limit shoudl it be $ (\phi | i \frac{dA}{dt}|\phi ) $
Jan
31
comment Euler-Maclarurin summation formula and regularization
thanks :) however i would more interested in regularizing divegent integrals $ \int_{0}^{\infty}x^{a}dx $ from zeta regularization $ \zeta (-a) $
Jan
26
comment How can one find this limit
take a calculatro and you will check that tends to 0 :D because of the exponential below :D
Jan
26
comment Evaluating $\lim\limits_{x\rightarrow \infty} \frac{f(x)}{x}$ knowing $\lim\limits_{x\to\infty} f(x+1) - f(x)$
for big 'x' $ f(x+1)-f(x) \sim f'(x)$
Jan
23
comment Limit of $\frac{1}{x}\ln{\frac{\exp{x}-1}{x}}$
i have used my calculator and obtained $ 0.49.. $ with $ x=0.0001 $ so the limit must be $ 0.5 $ :D
Jan
21
comment what is the period of this sinusoidal function funtion
we know that $ cos(u+2\pi ) = cos (u) $ for every 'u' so set $ u= \frac{4n \pi}{31}+ \frac{\pi}{5} $