# Rahul Narain

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bio website location age 28 member for 2 years, 9 months seen 10 hours ago profile views 3,195

Almost all of the questions on the front page these days are homework questions or textbook exercises. I think I'll be spending a lot less time here.

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 Mar11 comment Differentiation in 3d of sin root and fractions in one! -> to find the normal to a functionIf you say so, but there is a big difference between the PDF's $\left(-\frac{\partial f}{\partial x},1,-\frac{\partial f}{\partial z}\right)$ and rlgordonma's $\left(-\frac{\partial f}{\partial z},\frac{\partial f}{\partial x}\right)$... Mar11 comment Differentiation in 3d of sin root and fractions in one! -> to find the normal to a functionIt's probably not a great idea to ask strangers on the internet for help if you have no way to tell whether the answers you get make sense for your question. If I give you an answer that contradicts the existing one, how will you know which one is right? Maybe rlgordonma misinterpreted and gave an answer to a different problem. Maybe I'm the one misinterpreting your question. You need to go back and check some basic properties that the normal vector you want ought to have, so that you can look at the answers and say, "Thanks, that looks right", or, "That doesn't make sense because..." Mar11 comment Math hack for solving system of equationsAlso consider $\begin{bmatrix}-10^{-5}&1\\1&-10^{-5}\end{bmatrix}$... Mar11 comment Math hack for solving system of equationsThis sounds like a really bad idea. Take the matrix $\begin{bmatrix}0&1\\1&0\end{bmatrix}$ and compare the LU decompositions you get with pivoting and with your approach without pivoting. Mar11 comment Notation: Concatenate two functions (piecewise) / Concatenate two vectors, lists or tuples@user4514, this is the notation for vector concatenation. In linear algebra, $[\mathbf x\ \mathbf y]$ means nothing but the vector $[x_1\ x_2\ \ldots\ x_m\ y_1\ y_2 \ldots y_n]$, because in linear algebra we do not have nested lists. Mar11 comment Graph of a curveWrite $z=x+iy$ and expand $\lvert z+z^{-1}\rvert^2=2^2$ to get a quadratic in $x^2$ and $y^2$, then apply the quadratic formula to solve for $x^2$. Mar11 comment Is all algebraic commutative operation always associative?Averaging two numbers. Mar11 comment Correct way of saying that some value depends on another value x only by a function of xYou could say that $f(x)$ is "determined" by $\hat x$. That might not be clear to everyone, so you should spell out that $f(x_1)=f(x_2)$ whenever $\hat x_1=\hat x_2$. For example, if two random variables have the same distribution, then they have the same expectation. Mar11 reviewed Leave Open Fourier Transform of a function under an arbitrary coordinate transform Mar11 comment Graph of a curvei.stack.imgur.com/fOreA.png Mar11 comment Mean matching sizeHmmm, the tooltip for the downvote button does say "This question does not show any research effort..." Mar11 awarded Autobiographer Mar11 comment A basic question about iteration methodsJori's answer is right. To paraphrase Hurkyl's comment on another thread, there's always an easy way to reduce the number of iterations: each iteration, you do two steps of some other algorithm. :) In the end, what matters is the total compute time it takes to achieve a given accuracy. Mar11 reviewed Close Linear Transformation Question (Finding Standard Matrix and Image) Mar11 comment Ratio of primes $(x^2+x+(5+6m))$ to $(x^2+x+(3+6m))$The next time you edit the question, please also change the title so it does not consist entirely of math. Mar11 comment How to solve this coupled 2nd order Differential equation of a double pendulum- Runge Kutta methodPlease consider formatting your math using these instructions to make it easier to read. Mar11 revised How to solve this coupled 2nd order Differential equation of a double pendulum- Runge Kutta methoddeleted 2 characters in body; edited tags Mar11 comment Points on a plane@Berci: Can you explain why you tagged this graph theory? Mar11 comment Graph theory problem (edge-disjoint matchings)You're offering a bounty for someone to do your homework? Mar11 comment How does one verify if a vector is really recovered?Can you give some context or examples for the benefit of people not intimately familiar with compressed sensing?