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# John D'Errico

### Retired

4 909 total contributions since 2006

(Please don't contact me. I won't answer questions.) A retired mathematician, numerical animalist, woodworker, bridge player.

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How to construct the coefficient matrix（Pentagonal matrix） of five-point difference scheme?
Ok. I'll give you a hint. Constructing a pentagonal matrix STARTS with the function spdiags. A real virtue of spdiags is you wil...

environ 16 heures ago | 0

Multi-variable interpolation, grids and 2D guessing
Yes. you make yourself clear. This is a nonlinear optimization problem. Sometimes it might be called a calibration problem. Ac...

1 jour ago | 0

Fit experimental data to 1D convection diffusion solution
If your goal is to find an anaytical solution, of the form u(x,t), you may need to do some reading. Here, for example, a general...

1 jour ago | 0

find equation of plane intersect
Ok, first, the solution set is NOT a line. It is a curve, that lives in the (x,y) plane. Next, There is NO plane of intersection...

2 jours ago | 1

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Tight surface meshing of 3D points
Sadly, there is no optimal amount of data, no optimal distribution of points. Only as much data as you can get. The quality of...

3 jours ago | 0

finding and removing outlier
Your data does not appear to be normally distributed. Why not? There are no negative values, even though much of your data lies ...

3 jours ago | 0

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random close numbers within an array
I would choose a random integer from 1 to 10000. We can use that as a linear index into the elements of your array, as if the ar...

4 jours ago | 0

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Substitute variable and find maximum of the function
Yes. It is probably best, IF you have some completely general function where you won't tell us the function, to use GA. Ameer is...

4 jours ago | 2

How to select non-consecutive values from an array, or generate non-consecutive values?
I'd argue that requiring that the outliers/deviants NOT be consecutive biases your results. But this is easy enough to do. St...

4 jours ago | 1

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Second derivative from a smoothing spline fit
It is important to recognize that IF you have the curve fitting toolbox, then there are various ways to accomplish that you were...

4 jours ago | 0

i have a problme
What version of MATLAB are you using? This may be important, since strings in MATLAB only recently allowed double quotes, and th...

4 jours ago | 1

How can I index this definite Summation notation?
So many things wrong with the code you wrote. Invalid MATLAB syntax. Just wrongly implementing the formula in multiple places. ...

4 jours ago | 0

Finding odd and even values without functions
A = 1:10; A == fix(A/2)*2 ans = 1×10 logical array 0 1 0 1 0 1 0 1 0 1 As long as A is composed of...

5 jours ago | 0

the purpose of %f
They seem identical to me. See? The result is exactly the same. ;-) sprintf('%d',pi) ans = '3.141593e+00' sprintf(...

5 jours ago | 0

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How to measur the inclination angle of a palne?
Is there a way? NO. Sorry, but absolutely not since two points do not determine a plane. There are infinitely many planes that ...

5 jours ago | 1

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Whats wrong with this 45 degree line on the plot?
Appearances can be deceiving. Why do you think the line is NOT at 45 degrees? (I know, silly question, but there is a point in i...

5 jours ago | 1

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How can I find zero root for fifth eigenvalue of a matrix
Sadly, the "fifth" eigenvalue of a matrix has no meaning, since the eigenvalues are not assured to arrive in any special order. ...

5 jours ago | 0

Constrained polynomial regression with the constraint in coefficient
You are trying to estimate alpha, given a set of data points in the form of (x,y) pairs? Where alpha enters into BOTH coefficien...

5 jours ago | 1

Different ways to find limits?
So my response of a simple, succinct "Yes" is not enough? Drat. It did answer the question as asked. Actually, I was thinking a...

5 jours ago | 1

Defining Triangular mesh with delaunay and inpolygon ?
Easy peasy. And there is no need to bother with inpolygon here. [x,y] = meshgrid(0:.1:1); xy = [x(:),y(:)]; xy(xy(:,1) > xy(:...

5 jours ago | 1

How to convert string "26/7" into a double variable?
Whilst I am one of the people who tends to equate eval with evil... str = '22/7'; eval(str) ans = 3.14285714285714...

5 jours ago | 1

Matlab Solve a system of equations for a set of data
You get huge numbers, because you are trying to use symbolic tools here, but with double precision numbers. See what happens her...

6 jours ago | 1

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Composite Boole's rule
Looks like you made a serious effort. First, let me find Boole, not something even most mathematicians would remember off hand. ...

6 jours ago | 1

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How to find intersection between line and curve
Just because the only tool you own is a hammer does not mean every problem must be treated as itf it is a nail. Similarly, just ...

6 jours ago | 0

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Optimisation algorithm ga and fmincon
Athul has already explained it pretty well. Let me try to expand on that, to add some ideas or possibilities. Many nonlinear p...

6 jours ago | 1

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Getting weird results when calculating an integral using rectangle method
Look carefully at what you did. vec1 = pi/180*(0.005:0.01:14.995); Now, what is the stride between elements of that vector? Wh...

7 jours ago | 1

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Finding a number with conditions while using for loop
Why use a loop? ;-) Lets see, the smallest INTEGER with the desired property is 14404. We can get that as: n = 120; >> m = n^...

7 jours ago | 1

Portfolio Optimization in support of Monte Carlo method.
You cannot use a tool like fmincon to optimize a stochastic function. That is, any function where repeated calls to the function...

7 jours ago | 0

inverting variables with two matrices
Too late to be of any value to the OP, but others may have the same question... In general, if you have LINEAR surfaces, thus p...

7 jours ago | 0