John D'Errico

Retired

Last seen: Today | Con actividad desde 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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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)

alrededor de 6 horas hace | 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...

alrededor de 7 horas hace | 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...

alrededor de 18 horas hace | 0

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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...

2 días hace | 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...

3 días hace | 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...

4 días hace | 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...

4 días hace | 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 ...

4 días hace | 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...

4 días hace | 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 ...

4 días hace | 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)...

7 días hace | 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...

7 días hace | 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...

7 días hace | 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 ...

7 días hace | 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...

8 días hace | 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...

10 días hace | 0

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code for check wether the given differential function is function g(x) or g(x,y) in matlab
Easy. For example... syms x y expr1 = 2*x; expr2 = x + x*y; Now use symvar. symvar(expr1) numel(symvar(expr1)) symvar(exp...

10 días hace | 1

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can any one help me to find simple empirical model (equation) with a little parameters for this data
As an alternate model, one that employs the assumption that the curve passes through the point (0,1), can we find a different fi...

10 días hace | 0

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using global variables to avoid passing by value
Sorry. I'll call it a bad idea, generated because you erroneously think you are doing something good. MATLAB does not actually ...

10 días hace | 1

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is there a way to make the 0 x and y axis bold?
It seems simple enough to me. If you want to have ONLY the x and y axes emboldened... % arbitrary example plot, and there is no...

10 días hace | 0

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can any one help me to find simple empirical model (equation) with a little parameters for this data
Why not try it? This seems adequate: x=[31.96 30.25 28.27 26.84 25.4 24 23.27 21.89 20.85 18.6 17.02 16 15.81 13.6 12 11.19 9....

11 días hace | 0

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How to determine sets with only unique values across the columns of a large logical array?
Are you asking for ALL pairs that share no common elements? That will probably require loops. But I would not use a double loop....

11 días hace | 0

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interp1 fails to interpolate on a non-one-to-one function (curve)
interp1 does not fail here. It merely fails to do what it was not programmed to do! Interp1 does as designed. That you try to us...

12 días hace | 2

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Given a contour plot is there a function which returns a matrix corresponding to the density of lines at a given (x,y) coordinate?
Is there some specific function? No. But what does a high density of contour lines mean? It says the gradient is large in that v...

13 días hace | 3

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The sufficient and necessary conditon of fmincon funciton can find out the global optimal solution but not some other local optimal solutions
I'm sorry, but no. Fmincon CANNOT be assured of always finding a global solution. In fact, I can think of many ways it will fail...

14 días hace | 1

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Answers Not Running Code?
Now working. Many thanks for fixing it. As a test to verify that claim, I leave you with this complex mathematical computation. ...

15 días hace | 0

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find the way to transfer 2D projection back to 3D
The projection you used into the nullspace was correct, AND robust. However the inverse operation is less easy to perform. You n...

15 días hace | 0

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How to solve multiple equation with several conditions in it?
Is chi known? If not, then chi is one of your unknowns, and it must be solved for. Regardless, there will be infinitely many sol...

16 días hace | 0

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case sensitivity issue of "load" function
You cannot change the behavior of existing supplied code. If it works that way, then it does, and there is no way to tell load t...

17 días hace | 0

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I need to input a matrix of size 2x2 which is something like this A= [ 1 - 1.8629z-1 + 0.8669z-2 , 0 ; 0 , 1 -1.8695z-1 + 0.8737z- 2 ] which is a 2x2 Matrix
So do it. What is the problem? If z is known, then just type it in, pretty much as written. Do you know how to raise a number ...

18 días hace | 0

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