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John D'Errico


Retired

Last seen: 1 day ago Active since 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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Feeds

Answered
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)

20 hours ago | 0

Answered
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...

21 hours ago | 0

Answered
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...

1 day ago | 1

Answered
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...

3 days ago | 1

Answered
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...

4 days ago | 0

| accepted

Answered
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 days ago | 0

Answered
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 days ago | 0

Answered
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 ...

5 days ago | 0

Answered
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...

5 days ago | 1

| accepted

Answered
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 ...

5 days ago | 1

Answered
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 days ago | 1

Answered
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...

8 days ago | 0

Answered
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...

8 days ago | 0

| accepted

Answered
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 ...

8 days ago | 1

Answered
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...

9 days ago | 2

| accepted

Answered
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...

11 days ago | 0

Answered
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...

11 days ago | 1

| accepted

Answered
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...

11 days ago | 0

| accepted

Answered
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 ...

11 days ago | 1

| accepted

Answered
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...

11 days ago | 0

Answered
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 days ago | 0

Answered
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....

12 days ago | 0

Answered
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...

13 days ago | 2

Answered
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...

14 days ago | 3

Answered
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...

15 days ago | 1

Answered
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. ...

16 days ago | 0

Answered
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...

16 days ago | 0

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Answered
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...

17 days ago | 0

| accepted

Answered
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...

18 days ago | 0

| accepted

Answered
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 ...

19 days ago | 0

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