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Questions tagged [equation-solving]

Questions on the analytic and numerical equation solving functions of Mathematica (Solve, Reduce, NSolve, FindRoot, DSolve, RSolve, etc.).

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Solving a system of equations with constraints in specific order

This code gives about 1000 solutions for the variables a,b,c,d,e,f,g,h: ...
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3answers
175 views

Find the 3D region containing the origin bounded by given planes

I'm writing a code to generate the Wigner-Seitz cell of the reciprocal lattice for a given set of lattice translation vectors. For example, consider the Body Centered Cubic (BCC) lattice whose basis ...
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1answer
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Non-decimal representation of solutions of cubic equation

I am trying to solve a cubic equation as follows Solve[2 (Sqrt[x])^3 - 1.5*x^0.5*vd == 2 (Sqrt[\[Eta]0])^3, {x}] I want to get the solution of $x$ in terms of $\...
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Cannot solve for an unknown in an equation with exponents

I have an equation I need to solve for t. I have tried the code below: ...
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143 views

Can ThermodynamicData be used with NSolve?

Let's say I define a function giving the enthalpy of water: ...
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Solve linear system where unknowns are functions

I have the following linear system ...
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Solve doesn't find solution after harmless modification

I have a system with 5 equations and several parameters and Mma 11.3 finds a solution after a few minutes. But if I make a harmless modification, by inserting (kfus*wtss/kfis+1) in front of the two ...
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FindRoot for a transcendental function (PolyLog)

I am trying to solve an equation involving polylog using FindRoot, I am not sure how to give the initial guess as I want to solve the equation for a variety of parameters. Here is my code: ...
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31 views

Error message in NSolve

Can anyone help me to understand what I should add here to make sure no error message is shown? the code is: ...
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Precision, accuracy and very large/small exponents with Reduce

Numerical precision/accuracy question here. I am currently working on solving nonlinear equations with Reduce. Oftentimes, ...
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Different output to Solve command in Mathematica changing a variable for a subscripted variable

I have a system of equations that is solved with the Solve command in Mathematica 10.2, when one of the variables is changed for a subscripted variable the answer is not the same, the difference is ...
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Solve multivariable inequlity and read the output correctly

Hi I need to solve an inequality for multiple variables, and I do not understand in a good manner the results. The inequality that I tray to obtain is $$ a+b+(2*c)+\sqrt{(-2\,c-b-a)^2-4\,(2\,c\,b+b\,...
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Plotting a transcendental equation [duplicate]

I would like to get this graphic: from this article. Here $\kappa$ and $\varepsilon_{dd}$ are related by: ...
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find gravity acceleration (g) using FindFit

I need some help again, with this simple exercise: Use the data to determine g, but this time use the functional form T = 2 pi Sqrt[L/g] for the period T of a ...
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2answers
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Help explain failure of Solve method to solve economic equation

Below is an economic equation consisting of Y, Ki, Alpha, G, Ki, K, and two parameters Epsilon and Eta. Mathematica's Solve method computes G, in terms of the other variables, but fails to solve for ...
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Sum not recognized as a linear operator by Solve

When I try to solve an equation with a constant under the summation sign, Mathematica does not factor the constant out of the summation and fails to solve a simple equation. How do I make ...
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2answers
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Why Solve works but FindRoot needs precision. How to fix?

So how do I change the machine precision so that FindRoot works? I am unsure what to put into the program to fix. I tried setting the precision and that did not seem to work for me. ...
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Root (#1^3 …) How exploit the result of Matrix Eigenvalues [closed]

I try to determine the Eigenvalues of matrix[4x4] (algebraic results) but I don't understand the result. How exploit the result with #1^3 ? I would like find a algebraic result for each eigenvalues. ...
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How to Solve a Differential Equation System Depending on Variable Solved in an Implicit Function?

I have an ordinary differential equation system need to be solved by using Mathematica. $\frac{dk(t)}{dt}=((1-\kappa(t))k(t))^{\alpha}(1-\frac{\kappa(t)}{\kappa(t)+\frac{1-\alpha}{\alpha}\frac{\eta}{\...
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Kuhn-Tucker Conditions Take Forever

I am trying to solve a Kuhn-Tucker constrained maximization problem. The variables of interest are d1,d2,dr and i. The other symbols are parameters which I have several restrictions for. It's an ...
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Trying to solve a system of equation with a subscripted variable with Reduce

When I run Reduce[expr, vars] in which one of the variables has a subscript, Mathematica gives me an output after some minutes (the system has 10 equations), but if ...
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3answers
78 views

Checking if two expressions are related

Is it possible in mathematica to check if two expressions may be able to be written in terms of the other? The expressions I had in mind were of the form $$\alpha = \frac{1-x}{1-y}\,\,\,\,,\,\,\ \beta ...
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Why do I get a blank graph when using Manipulate with NDSolve?

I'm trying to model an SIR model using the Manipulate function so that I can change the parameters. Although, when I enter in the following code: ...
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1answer
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Find x at a given t and f(x,t) == constant from a system of coupled PDE

Is it possible to find the speed of a moving front (produced by the below code) in which modulating beta changes the speed of the front? Basically, I am trying to map the x-values of f(x,t)=0.5 ...
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1answer
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Implicit piece-wise function in derivative for differential equation solver

I want to set up a model/simulation that involves numerically solving a set of coupled differential equations, but at some point in time I want some of the variables to quickly jump to a constant ...
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Why doesn't Mathematica find solution for differential equation when Wolfram Alpha does?

I need to solve differential equation: dy/dx=tan(1/2*(Pi/2+arctan((|3tan(35*Pi/180)-x|)/(y+3)))), y(0)=0 Wolfram Alpha gives me the output in shape of plot and I need to get the formula used by ...
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Solve is taking days

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1answer
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Why is NDSolve not working for the given system?

I'm trying to find the solution curves for a system of differential equations. In order to try to complete this task, I've attempted to use NDSolve, but I get an error that says that there are fewer ...
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1answer
114 views

Finding the maximum real part of roots

Suppose that I have this problem ...
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1answer
37 views

Missing solutions of a matrix congruence

There are some indications that the following matrix congruence might have a one-dimensional family of solutions. Also this discussion is relevant ...
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1answer
83 views

FindInstance and cosine system of equations

I'm intrigued by Mathematica indisposition to find an example of the following system of equations $$ \cos(x) + \cos(y) + \cos(z) = \lambda_1, $$ $$ \cos(x+u) + \...
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2answers
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How to solve for variable inside of Re[ ] function?

I am trying to solve for x in the following equation NSolve[ c == Re[ Sqrt[ (((a - b I) x^2) / ((a - b I) - x^2)) ] ] , x ] It seems to be too much for NSolve.
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2answers
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Both DSolve and NDSolve fail for a simple system of two PDEs

Using Mathematica 10.4.1, I'm trying to solve some simple (I think!) partial differential equations with a form more or less like this: $\qquad \frac{\partial v}{\partial t} = b v\;, \quad \frac{\...
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42 views

Transcendental equation analytical solving

I want to solve a transcendental equation analytically, the equation basically is ...
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1answer
65 views

Produce result faster than “Table[If[…]]”

I would like to generate a list of output satisfying some condition for instance an equation but faster than the following code: ...
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2answers
129 views

Eigen value solution of coupled ODEs

I want an eigen value solution of following coupled ODEs: But the code showing errors. ...
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1answer
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Problem with solving a system of equations based on a matrix

I am trying to solve a system of equations based on a matrix in order to find the "stationary" matrix. the code I've written should break the matrix down into a system of equations such that some ...
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1answer
61 views

NDSolve ERROR: There are more dependent variables, than equations, so the system is underdetermined

I have a question concerning NDSolve and a system of differential equations. I have 4 variables and 4 equations. Why do I get this ERROR :There are more dependent variables, {f[j,p],U[j,p],(f^(0,1))[...
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1answer
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NDSolve outcome depends on system size?

I tried to solve a simple wave equation numerically and plot the result within the time range t=0...1. Now when I change the tmax in the NDSolve the outcome of my plot (I don't change anything else ...
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`FindRoot [ ]`::jsing: Encountered a singular Jacobian at a point…WHY

I have a large system of linear and nonlinear equations. I know that the system is not singular at the point given because I calculated the Jacobian and evaluated it at a given point. The resulting ...
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1answer
81 views

Problems Minimizing a Function

I need to minimize a function, but it takes a long time. So I don't know if it's normal or if there's something wrong with my model. ...
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1answer
782 views

Mathematica 3D Heat Equation Solution

I've been working on trying to analyze the Heat Equation in water both experimentally and theoretically. The model goes as: there's a cuboidal bath (of say, 15x7x5 inches) filled with water, and an ...
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2answers
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Solving 2D Reaction-Diffusion with Explicit Euler Method

I am trying to solve this following 2D-Diffusion equation (I wrote it in Word, so it looks nice) (Note: I am solving this for a 100x100 mesh point) I tried to solve the above equation using the ...
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Warning: Reduce was unable to prove that the solution set found is complete

I am trying to solve the below equation, but there is a warning " Warning: Reduce was unable to prove that the solution set found is complete" . Could somebody please introduce a method for ...
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1answer
33 views

Solve an equation for a specific unknown? [closed]

I have this equation: ...
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74 views

How to speed up FindRoot?

How can I speed up my FindRoot time, for the below code? It takes 25 seconds on my machine to find one single root for a set of $g$, $mx$, $m\psi$ values, I need to ...
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1answer
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Exploring an equation's solution space with Manipulate [closed]

I have the following simple Manipulate code that I use for solving an equation in two stages. In the first stage I identify the initial values ...
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1answer
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Solving an equation with 3 free parameters in a way that satisfies a condition

I have an equation in terms of $q$, $c_2$, $c_3$ and $r$. I just know that $q$ and $r$ are positive (there are no restrictions on $c_2$ and $c_3$). ...