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John D'Errico


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Retired

5 311 total contributions since 2006

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

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Answered
How to find all solutions to a system of two nonlinear equations?
Here is the rub - you are in trouble IF you really want to find ALL solutions of a system of totally general nonlinear equations...

ungefär en timme ago | 0

Answered
inverse matrix calculation problem
You need to understand that the matrix a=[1 2 3;4 5 6;7 8 9] is singular. Therefore it has no inverse. You may as well have s...

en dag ago | 0

Answered
Matlab code for fractions
Confusing question. Do you know the value of s? s = 7 s = 7 >> 1.5/s ans = 0.214285714285714 >> format rat...

2 dagar ago | 1

Answered
Direction between points from [0,360]
The function atan2d gives angles that are from -180 to 180. Not a problem. This works: mod(atan2d(y,x),360) But you want the a...

4 dagar ago | 2

| accepted

Answered
PLEASE HELP ME CONVERT THIS EQUATION TO MATLAB CODE
Well, you did make an effort, and it was pretty close. But did you read my response? Did you read the help for tand and cosd? W...

5 dagar ago | 0

| accepted

Answered
Exponential Piecewise function creation.
A spline is not a piecewise exponential function, so using spline directly would be a waste of time, IF you truly need a piecewi...

5 dagar ago | 1

| accepted

Answered
minimum value of one inequality
This is just mathematics. r = 0.001; s = 7; n ALWAYS appears as 2^(-n/2). So calling that number x, first write the problem ...

5 dagar ago | 4

| accepted

Answered
Generate random sequence number with condition
I think you still do not appreciate that you cannot just say something is "random" and not define the distribution. For exampl...

6 dagar ago | 1

Answered
error using cross (A and B must have at least one dimension of length 3.)
I'm not sure what you want in terms of a 25000 dimensional cross produt between two vectors. What do you think it means? After a...

6 dagar ago | 1

| accepted

Answered
Diagonalising a Skew-Symmetric Matrix
Matrices that are defective will not have a complete set of eigenvectors. https://en.wikipedia.org/wiki/Defective_matrix " In...

7 dagar ago | 0

Answered
Molecules on flat surface by using the package of Atomistic Topology Operations
Without more carefully reading the help for the code you are using, when I go to that link, it describes Box_dim as a variable. ...

7 dagar ago | 0

Answered
Taylor polynomial of a not infinitely differentiable function.
Your question has absolutely nothing to do with MATLAB, but you seem to have made some effort. So I will point out that you may ...

7 dagar ago | 1

| accepted

Answered
Numerical integration over (0, x1)
If t is unknown, then you need to use symbolic tools to do the integration. syms x t K = 1/((exp(0.014342/(x * t)) - 1)) * (1/...

8 dagar ago | 1

| accepted

Answered
How can i compute an interpolated curve?
You don't provide the actual data. But you are asking to do something which is trivially easy for the eye to do, more difficult ...

8 dagar ago | 1

| accepted

Answered
how can I draw half circle in matlab ?
Sigh. You DID make an effort. It is not even a terrible solution, though what you did will not work, since it will at best be a ...

9 dagar ago | 2

| accepted

Answered
linprog cannot find solution
While you claim a solution exists, initially, I sincerely wonder if it truly does. I may learn differently by the time I am done...

9 dagar ago | 1

Answered
How to Run PSO with Real Data
You can't. That is, the optimal is just the optimal value at each point in your sample. You cannot do better than that, at least...

10 dagar ago | 0

Answered
How to solve for x in an sin equation?
No. There are INFINITELY many solutions. So you cannot say you want the OTHER solution. You can simply enough find a positive so...

11 dagar ago | 0

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Answered
linearly independent eigen vectors
And for some strange reason, you think that eig as applied to a diagonal matrix does not produce a set of independent eigenvecto...

11 dagar ago | 0

| accepted

Answered
How can large sparse matrix (Y bus matrix of order 800*800) be done inverse in Matlab?
An 800 by 800 matrix is barely what I would call large anymore. Perhaps 40 years ago, yes. Today, it is almost tiny. Ok, not tin...

11 dagar ago | 0

| accepted

Answered
How to subtract from a vector array and, if the answer is lees than 0, have it return a value of 0
Simple. No need to test anything either. Just use the max function. x = [1 2 3]; x(1) = max(x(1) - 2,0); x x = 0 ...

14 dagar ago | 0

Answered
Calculate factorial with 2 variables
Why do you feel you need to use loops? You should never write your own code for something that is far better computed using an e...

14 dagar ago | 2

Answered
Random number from 1 to 100 if the number between 20 to 30 add 100 if the number 50 to 60 minus 100.
I'll generate 10 samples. N = 10; X = randi(100,1,N); ind = (X >= 20) & (X <= 30); X(ind) = X(ind) + 100; ind = (X >= 5...

16 dagar ago | 0

Answered
Curve Fitting and Distribution Fitting
You have what, 13 data points? A model with many exponents in it? A model that is surely based on no real physical meaning of th...

18 dagar ago | 0

Answered
Integral of bessel function returns all zeros
It is a really bad idea to use gamma as a variable name because you are now stepping on the name of a very useful function in MA...

18 dagar ago | 0

| accepted

Answered
Sampling data using contraints
Use my randFixedLinearCombination tool, as found on the file exchange. (I did just post an update today, fixing a bug the code t...

18 dagar ago | 1

| accepted

Submitted


randFixedLinearCombination
Uniform random samples over a n-dimensional hyper-rectangle, subject to a linear equality constraint

18 dagar ago | 7 downloads |

Answered
recursive function to check a Palindrome
Why not try? I won't write it for you, as the entire problem is to make you think. I'll give you one hint, and I will hope nobod...

18 dagar ago | 1

Answered
Constructing a Maze with vectors
Insert NaNs between eack block of lines. X = [x,nan,x1,nan,x2]; Y = [y,nan,y1,nan,y2]; Now the various sets of line segments ...

19 dagar ago | 1

| accepted

Answered
How to solve exponential equations.
Thank you for posting this as a question instead of an answer. The simplest way to solve this is if you have the curve fitting ...

19 dagar ago | 0

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