Jose Garcia
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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 $u= \frac{4n \pi}{31}+ \frac{\pi}{5}$