John D'Errico

Retired

Last seen: Today | Actif depuis 2006

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A retired applied mathematician, consultant, sometime mechanical engineer, numerical analyst, woodworker, bridge player. Please DON'T contact me, as too many students then decide I am their personal consultant if I allow it. I won't do consulting by mail.

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How to find the point that resides on a contour that is closest to a known point?
You can't. That is, you cannot use calculus to do this, at least not as it is, not directly. That contour is a set of piecewise ...

environ 7 heures il y a | 0

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i want to solve 3 non linear equations in matlab
You can want anything you want. I, for example, want to see peace in the world, with everyone treating everyone else as an equal...

environ 9 heures il y a | 0

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I need the installer for Matlab R2010b SP1
We cannot help you. Answers does not have software, cannot supply it. The only people who would be able to help you are at MathW...

environ 22 heures il y a | 0

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problem with genetic algorithm
Clearly, your constraint function is not evaluating as a real number. It is likely complex, based on the error message. (Though ...

4 jours il y a | 0

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fsolve stopped because the last step was ineffective
This is a common mistake I see being made. You have a problem that has symmetries in it. x(1), x(2), x(3) all play similar parts...

5 jours il y a | 1

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Accessing a vector form a matrix
As an example, given vectors x1,x2,x3. I'll make some up. x1 = randi(10,1,5) x2 = randi(10,1,5) x3 = randi(10,1,5) Combine t...

6 jours il y a | 0

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Find the variable inside the LCM argument
You have the expression: B=lcm(nrt,Nr*Nc) where the variables B and nrt are known. The unknown variable is the product nr*...

6 jours il y a | 1

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volume calculation from the DelaunayTri(x,y,z) function
Its not THAT hard. You compute the volume of each simplex in the tessellation. Sum the absolute values up. (Since those simplexe...

7 jours il y a | 1

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is there any reason for this?
Is there any reason? Well, does there absolutely need to be a "reason"? These are a set of choices, made over a period of multip...

7 jours il y a | 1

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find a zero of a two-variable function
A two variable function is a surface. (@Sam Chak said this, but did not take the idea to the point of completion.) But if a sur...

8 jours il y a | 0

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Extracting values from optimization variable and setting constraints
Are you saying you want to constraint the sum of absolute values? That is, in context of a LINEAR programming problem? This is ...

9 jours il y a | 0

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I am trying to do multi step math and keep returning the wrong answer
Of course it is wrong. Why did you write 72/13? Why 25/3? I think you don't understand that 8(1/3) is NOT the same thing as ...

9 jours il y a | 0

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Calling GA() with 10 generations 10x vs GA() with 100 generations.
Is there any difference? Of course! Make it more extreme yet. Suppose you were to stop after 1 iteration. ONLY 1. But do it 100 ...

11 jours il y a | 0

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Check the result if it is divisible or not
Do you understand that you are using DOUBLE precision arithmetic to compute that mod? Do you understand that 7^2026 is a number ...

11 jours il y a | 1

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Treating ties in a Gaussian Elimination process
Um, max does exactly that. Where is there a problem? x = [1 2 3 4 3 4 1]; [xmax,ind] = max(x)

12 jours il y a | 0

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Matlab editor settings: notify undefined variables
For one example of a scenario where a variable appears to be undefined, yet is properly used, consider the case of a nested func...

12 jours il y a | 0

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Trying to determine roots of this polynomial (Det)
k = [1750 -750 0; -750 1250 -500; 0 -500 500]; m = [75 0 0; 0 75 0; 0 0 50]; D=inv(m)*k; syms x I = [x,0,0;0,x,0;0,0,x]; La...

13 jours il y a | 1

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Area using trapezoidal rule
Um, this is WRONG. Basic integral calculus does NOT say a result should never be negative. You are possibly misunderstanding wha...

14 jours il y a | 1

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Incorrect Coefficients from Symbolic Polynomial Equation Solving
NO. You think there is a unique solution. But that is wrong. syms a1 a2 a3 a4 x; % Define symbolic variables % Define the equ...

15 jours il y a | 0

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How do I extract one point from a piecewise function?
You can define a piecewise function as a symbolic function. For example... syms t M_xy(t) = piecewise(t < 0, 0, t > 0, t); % N...

16 jours il y a | 0

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Why isn't my MATLAB plot not showing up?
A STRONG suggestion is when you do something, to look at the results of your code. Don't just give up when you see nothing in th...

16 jours il y a | 0

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Estimating SQRT of a non-negative number the long way
If you generate an error message, MATLAB stops execution. An error is a failure, and nothing gets past that. However, a warning ...

16 jours il y a | 0

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Finding the index k of square nilpotent matrix A
A nilpotent matrix is a square matrix A that has the property that for SOME integer power k, we have A^k is entirely zero. The s...

16 jours il y a | 1

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Simultaneous Constant Optimisation for Curve Fitting
Do NOT manually search through a 15 dimensional space for a solution. You are just wasting your time, and this is why tools are ...

16 jours il y a | 1

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How do I graph the negative portion of a square root plot?
Since in reality, you have what is called an implicit function, we want to use fimplicit. I might do it like this: fun = @(x,y)...

19 jours il y a | 1

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How to invert a square matrix using Gaussian elimination
I assume you are forced to use your own code for gaussian elimination here. A bad idea if you are not. NEVER write such code you...

19 jours il y a | 0

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Lognormal distribution parameters mu and sigma
In MATLAB, the tools that use a lognormal distribution have you provide mu and sigma in terms of the underlying normal distribut...

19 jours il y a | 0

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lsqlin unexpected results !!!
Unfortunately, you would have been better off using a direct least squares. That would allow us to see how well the calibration ...

19 jours il y a | 1

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Trying to find pi using when loops to a tolerance of 10^-4.
Many, many things wrong in your code. I'm amazed nobody else as pointed most of them out. It was a start in the correct directio...

20 jours il y a | 2

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how to use a variable in finite field
g = gf(3, 8) whos g g is a gf object. But g is not compatible for multiplication by a symbolic parameter. These are two indepe...

22 jours il y a | 0

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