Variable Definition and 1000 Threads

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.
This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

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  1. V

    Converting dependable variable to independant and vice versa

    Hello guys, First of all, thanks for looking at this post and trying to help me. I have function which is shown below, F1= (ε1-√(1+(ε1 μ1-1)x^2))/(ε1+√(1+(ε1 μ1-1)x^2)); F2= (ε2-√(1+(ε2 μ2-1)x^2))/(ε2+√(1+(ε2 μ2-1)x^2)); F=F1*F2; Here, F is a function of x --> F(x) I...
  2. mesa

    Trying to figure out a single variable expression for Sin

    Well, I am now trying to figure out a single variable expression for Sin. I have a couple ideas using some pieces of geometric formulas I have played with recently but this is still new to me. I'm not talking about sin x, but an algebraic expression for the sin wave. Any thoughts?
  3. M

    Designing a Cost-Effective Variable Frequency Generator for Microwave Cooking

    What I am looking for is a cheap, lightweight way to generate a range of specific frequencies. To expand a little, a microwave uses a constant frequency but may vary its power to cook the food. I've heard it tries to match the resonant frequency of the water molecule in order to agitate it...
  4. S

    Solving for Car & Biker's Meeting Point w/ Variable d

    Homework Statement A car is stopped at an intersection with a red light and a biker (in a bike lane) with velocity v is approaching the car. When the biker is a distance d from the intersection, the light turns green and the car begins to accelerate at a constant acceleration a. If the biker...
  5. D

    Natural Logs, adding variable exponents.

    Homework Statement e^2x * e^2 Homework Equations The Attempt at a Solution Really, I'm just a little bit confused on how I would go about adding the exponent. None of the examples in the book cover that. Is it simply e^2x+2?
  6. D

    Clarification of the independent variable for a partial derivative

    For some non-linear 3D function, let's say I want to take the partial derivative for x where y is constant. Each point for Z will be different of course since it's non-linear. So let's say I plug in an X of 3 where Y is constant for some function and I get a slope of 5 as my answer. Is it...
  7. R

    Variable Frequency Transformers

    How are described in the Industry those transformers able to operate with a variable frequency? These transformers will be used downstream of a Frequency Drive. Anyone knows? Any specific characteristics? Thanks
  8. S

    Finding Discrete Counterpart of Continuous Variable Energy Term

    Dear Sir… I am looking for a discrete counter part of a continuous variable. the continuous version of energy term in a liquid crystal is given by [\vec{n}\cdot(\nabla\times\vec{n})]^2. This is a square of a dot product between a vector 'n' and its curl field. My question is what is the exact...
  9. M

    MATLAB How to plot MATLAB contour plots for a variable z dependent on x and y variables

    Hi all I am trying to do a MATLAB plot, just like the one attached. DATA: Graph shows a parameter (suppose) humidity along the height from ground upto 6 Km. Instrument measures humidity at each 30 meter height from the ground. Sample is repeated after each 15 minutes. DATA FORMAT: Data...
  10. A

    Mathematica How to Define a Complete Bell Polynomial in Mathematica?

    Hello, how do you define a function or make a list with n elements, where n is any? More precisely there is a function in mathematica, BellY BellY[n,k,\{x_{1},...x_{n-k+1}\}] which gives the partial Bell polynomial. I would like to define in mathematica the complete Bell polynomial defined...
  11. P

    About the definition of discrete random variable

    About the definition of "discrete random variable" Hogg and Craig stated that a discrete random variable takes on at most a finite number of values in every finite interval (“Introduction to Mathematical Statistics”, McMillan 3rd Ed, 1970, page 22). This is in contrast with the assumption that...
  12. O

    How to find/prove this two variable limit

    It has been a while since I have done limis now, and I want to make sure I am doing this correctly. I have the problem listed below (x-y)/(x+y) as (x,y) -> (0,0) In my mind I can gather that it will asymptote as you approach from either (-1,1) or (1,-1) I also know that it will...
  13. S

    Solving for variable inside summations

    Hi, I have an equation: \frac{q-\bar{q}}{w_{i}^{2}+w'_{j}^2}=\sum^{n}_{i=0}\sum^{m}_{j=1}F_{0}\cdot a_{ij}\cdot cos(w_{i}\cdot x)\cdot cos(w'_{j}\cdot y) I'm trying to solve for a_{ij} I have the solution, but I'm not sure how they came up with it. The solution is...
  14. W

    Model predictive control variable Cost function

    Hi all, I have a MIMO system which I have developed in Simulink and I have also work out the Model predictive control for it. Now I have a manipulated variable which I want to be used during night and remained turned off during day. I have tried but could not figure out how to do this in...
  15. C

    Fortran Weird fortran problem (Assigned variable value doesn't match up with output)

    I ran into a kind of strange thing today while coding. I needed Pi in a program, so I used 2*accos(0.0) to pull it out, but my answers were slightly off. I checked the Pi calculated and it was off after a few decimal places. "Odd", I thought, and just decided to put in Pi as an assigned number...
  16. S

    Two variable limit question (Question/Answer included)

    Homework Statement Problem: Find the limit if it exists, or state that it does not exist for: lim (x,y)→(0,0) [2x^3 + 6y^3]/[x^2 + y^2]. Answer: lim (x,y)→(0,0) [2x^3 + 6y^3]/[x^2 + y^2] = 0 Homework Equations r^2 = x^2 + y^2 The Attempt at a Solution I know I can replace...
  17. QD311

    Center of mass variable ambiguity

    Homework Statement I'm trying to understand the equations given in my textbook (Principles of Physics, Ninth International Edition) for finding the center of mass. The equations are given below. They're used to find the x, y, and z components of the center of mass for objects. I'm trying to...
  18. A

    Chilled Beams vs. Variable Refrigerant Flow

    Can somebody give me a scenario when chilled beams would be the preferred method of conditioning and ventilation as opposed to the variable refrigerant flow systems? I have heard a lot about chilled beams, but I'm not sure they are ever going to really get popular in the US, because I don't...
  19. M

    Finding d/dx Given d/dr and a Change of Variable

    let x=r^2. if we have already d/dr, how to find d/dx? I was saying, d/dr=(d/dx)*(dx/dr)=(d/dx)*2r ? But am doubting it?
  20. M

    MATLAB Select variable automatically in matlab

    Hi, I want to select my variable automatically in MATLAB so I used the following commands: (may variable for example are: Mpoint1,Mpoint5,Mpoint14,...) for i=1:50 if exist (eval(['Mpoint' num2str(i)])); statements else...
  21. M

    3 Variable equation that i'm stuck on .

    Homework Statement Solve the following equation in N^3: x^2+y^2+z^2=xyz-1 The Attempt at a Solution I moved all the terms to one side, but I don't know what to do from there on.
  22. T

    Confusing in transforming variable

    Considering U1 and U2 which both follow Uniform (0 , A), then after the transformation of variable, i can find x=(U1+U2)/2 has f(x)=2/A(both my classmates and me get this result). However i am confused that when i integrate x by it range from 0 to A (because U1 and U2 both ranged from 0 to...
  23. M

    Calculating variable gain for difference amplifier

    Homework Statement http://img706.imageshack.us/img706/263/hwstatement.png Homework Equations KCL/Node voltage method Ideal op-amp rules The Attempt at a Solution I solved a previous, similar problem where the gain was fixed and the resistor mass going into the inverting and noninverting...
  24. I

    Calculating Force on a Man from Variable Mass Water Jet

    Homework Statement Water with mass density ρ are being shoot from a jet with cross sectional area A and velocity v0 at a man with mass m0. 1. Find the force acting on a man at rest (assume all the water are absorbed in the clothes) 2. Find the force acting on a man escaping at velocity v<v0...
  25. G

    Derivative of a function with variable also in the integrand

    F(y)=Integral(e^{-x^{2}y^{2}}dx between y and 0 (where y>0). I know you have to make substitutions with y, but then you have to make a further substitution along the way and that's where I'm a little lost. Can anyone help point me in the right direction? I know you can set u=-y^{2} and v=y in...
  26. J

    CDF of a variable with a negative exponent in its PDF

    Ok, this one's got me stumped! Let's take as an example the probability density function for a random variable X so that: f(x) = \frac{4}{3x^{3}} 1≤x<2 f(x) = \frac{x}{12} 2≤x≤4 f(x) = 0 So the CDF for this variable comes out as: F(x) = \frac{-2}{3x^{2}} 1≤x<2 F(x) =...
  27. K

    MATLAB Why is my variable not updating in my SIR model?

    I made an epidemics SIR model but want to vary one of the parameters but it's not working can someone help I pasted the code below I believe the parameter I want to change, v, is not updating: v_range=linspace(0,.1,1000); numinfected=0*v_range; for vi=1:1000; for klok=1:100000...
  28. P

    Calculating Work with a Variable Force

    Homework Statement An object is moved from 0 to 9 meters with a force varying according to the function F(x)= 50(5-square root x). What is the work done. Homework Equations The Attempt at a Solution I tried to take the integral and evaluate it from 0 to 9. I get 1350J no...
  29. M

    Solving a two variable equation

    Homework Statement For every (x,y) in Z^2 solve : 6x-5y=1The Attempt at a Solution I did 6x=5y+1 and then i said let x=5n+1 so then we get 6(5n+1)=5y+1 and that gives that y=6n+1 so the solution is (5n+1,6n+1). Is this the correct way of solving this?
  30. EVriderDK

    Cannot do the find command with undefined variable ?

    Cannot do the "find" command with "undefined variable" ? http://peecee.dk/upload/view/363439 As you can see, i have to make I_1:=r. I cannot directly write find(I_1,I_2,I_3)→ It will not do that. Why is that, and is it possible to force it? I cannot see the difference between using "I_1" and...
  31. J

    Continuous random variable (supply and demand)

    Homework Statement In the winter, the monthly demand in tonnes, for solid fuel from a coal merchant may be modeled by the continuous random variable X with probability density function given by: f(x)=\frac{x}{30} 0≤x<6 f(x)=\frac{(12-x)^{2}}{180} 6≤x≤12 f(x)=0 otherwise (a)...
  32. F

    Changing the variable in a differential system

    Homework Statement Hello everyone. I have differential system consists of: \frac{dx_1}{dt}=x_1(1-2x_1+x_2) \frac{dx_2}{dt}=3*x_2(x_1-x_2) I want to change variable using: x_i=a_iz_i i=1,2 where z is a unit vector and rewrite the differential equations Homework Equations...
  33. D

    Are My State Equations Correct for a Multi-Input System Using KVL and KCL?

    what I've posted is a question from a study guide for our test next week, on the attachment is my work. The professor only did SISO examples in class and moved really fast, I am not really sure if i found state equations correctly using KVL and KCL. Any help is appreciated
  34. K

    Limits for a truncated random variable

    Suppose that X is a random variable distributed in the interval [a;b] with pdf f(x) and cdf F(x). Clearly, F(b)=1. I only observe X for values that are bigger than y. I know that E(X|X>y)=\frac{\int_y^b xf(x)dx}{1-F(y)}. Moreover, \frac{∂E(X|X>y)}{∂y}=\frac{f(y)}{1-F(y)}[E(X|X>y)-y] I...
  35. N

    Variable selection CART and SVM?

    Hey, i am taking an applied statistics course and have a question related to an analysis of a data set. I am observing a great degree of positive correlation among the variables, and as expected I find that some of the variables are non significant when i apply general linear regression. I...
  36. T

    Fortran Fortran passing parameters to a function of one variable

    Hi all! I have a question about fortran. So far in my coding, if i wanted to define a function ( say f(x) ) I would manually set a grid for x, and then define f for each x value, so that i would have a vector for f and a vector for x. However, I'd now like to use the 'function' command, so i can...
  37. C

    Calculating the time taken for variable declaration

    Hey guys ^^, basically there's this problem I've come across and I am somewhat curious of your opinions on it. There's a bike that was previously on an incline that begins to run on a level surface. Assuming that the only resistive forces acting on the bike are due to aerodynamic drag I want...
  38. O

    Calculating Variable from a Derivative

    My answer for part (i) matches up with that of the book but solving for part (ii) has gone awry. Can anyone help me clear this up? Many thanks. Homework Statement Q. The given diagram (see attachment) shows part of the graph of y = x2. p(x,y) is a point on the curve & a(6,0). bp \perp...
  39. S

    R-Squared Difference when Dependent variable is $/lb vs Total Dollars?

    Why would I see a fairly significant difference in r-square (98.5% vs 85%) when I change my dependent variable from Total $'s to $'s per lb? My independent variable in both cases is weight. Each equation yields essentially the same answer in all tests of weight (within .08%). How do I know...
  40. Whovian

    C/C++ C++ array with variable no. of elements?

    #include <iostream> using namespace std; int a; int main() { cin >> a; int c [a]; for (int i = 1;i!=a+1;i++) { c [i] = i; } for (int i = 1;i!=a+1;i++) { cout << c [i] << endl; } return 0; } This compiles and does exactly what it's supposed to. But wait...
  41. W

    Control current despite variable resistance

    For a senior engineering class I need to design a device that, using a 5 volt source I need to deliver a 5mA current with a load that can vary from 500-100K ohms. Using transistors I know one can create a current limiting circuit, but once the resistance gets high enough (not sure how to...
  42. P

    Fortran Fortran: printing the Printing the Hex contents of a character variable

    Given a character variable character*10 name write (6,111) name 111 format (?) what do I put in the format statement to print the hexadecimal contents of name?
  43. T

    Distribution of sum of discrete random variable

    Edit: I have to think more about this, I'll post later.
  44. L

    2nd Order Linear homog ODE of variable coefficients

    Hey, Every where I look I can only find books and pdf talking about the uniqueness and linear independence of the solutions but I haven't been able to find a procedure of finding the solutions to one of these ode's if you haven't been already given a particular solution. I've been trying...
  45. K

    How Much Does a Spring Compress Under a 10-N Force?

    Homework Statement if a 10-N force is used to compress a spring with a spring constant of 4.0x10^2N/m, what is the resulting spring compression? Homework Equations The Attempt at a Solution
  46. V

    MHB Factoring when exponent is a variable

    I am trying to factor the following equation $$\large(x^{\frac{1}{n}}+a)^{n-1}$$ but the fact that the exponent is n-1 is throwing me off. How could I go about factoring out this equation? Thanks.
  47. I

    Finding Derivatives with Respect to a Variable

    Homework Statement The question says ti find the derivative of y with respect to the independent variable. The equation is: y=4 ln 2t. Homework Equations I know how to find derivatives using the product/quotient/chain/etc rules, but that isn't what the question is asking for. I...
  48. O

    Multi-Plate variable Capacitor Questions.

    Homework Statement A multi-plate variable capacitor has 4 pairs of plates. The plates, when closed, are separated in air by 0.01mm. A capacitance range of 10 - 400 pF is required a) estimate the required radius, R of each plate. b) The capacitor is set to the maximum of 400 pF and is...
  49. Spinnor

    Charged particle in a box, variable m and e.

    Say we have one spin-less electron in a box in the ground state. Say we have dials on the outside of the box that can vary mass and charge of the spin-less electron. Suppose we start with one spin-less electron in a box with m and e set to as near zero as we please, but not zero. As m and e go...
  50. GreenGoblin

    MHB Extrema points, function of three variable

    "Show that if a>b>c>0, then the function $f(x,y,z) = (ax^{2} + by^{2} + cz^{2})e^{-x^{2}-y^{2}-z^{2}}$ has two local maxima, one local minima, and four saddle points"
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