John D'Errico

Retired

Last seen: Today | 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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Simplify in GF(2)
In the help docs for simplify, we see: "S = simplify(expr) performs algebraic simplification of expr." The key word there is A...

3 hours ago | 0

Answered
Hello I am solving a lp problem and linprog shows the wrong answer. If I solve it with fmincon, I get the answer.
H= @(i,j) exp(-abs(i-j)); % anonymous function m= 4; noise_pow= 1e-9; g= [15; 12; 9; 6]; A= [-H(1,1), g(1)*H(2,1), g(1)*H(3,1...

5 hours ago | 1

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Cumtrapz integration drift with time
This is expected behavior. And reducing the time step WILL reduce the problem. And it IS something purely mathematical, yes, you...

1 day ago | 0

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Answered
Numerical search for the singular points of a complex matrix
I'm sorry, but the use of det for anything like this is a bad idea. How did they do this in 1972? Sigh, they used determinants ...

4 days ago | 1

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Vectorized code slower than loops
Read the response by @Steven Lord again, in case you missed it. (Actually, that is always good advice for any of his responses.)...

4 days ago | 2

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How to use "predict" to get x value for a given y value?
Huh? Of course polyfit works. It does NOT require the x values be distinct. I'm not at all sure why you think that is necessary....

6 days ago | 0

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What information is stored by p1/p2/... in this code sample?
Undocumented, uncommented code, from an unknown source? Shiver. No. It is not a distance computation. That much I can say. The...

7 days ago | 1

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Answered
Attempt to execute SCRIPT plot as a function,how to solve it?
Don't name your script plot. In general, don't use existing names of supplied functions for your own functions or scripts. You ...

7 days ago | 0

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How to identify such functions as addpath that are coming with MATLAB?
Follow two rules when you are writing or providing functions to run in MATLAB. Rule # 1: NEVER put your own functions in the Ma...

8 days ago | 0

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Are there MATLAB functions or scripts to perform boolean operations on triangulated solids?
Are there such codes in MATLAB? Yes. I wrote them. intersect, union. In 2-d and in 3-d. (Not sure if I wrote a setdiff variant. ...

13 days ago | 0

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Product of matrices over finite fields
Even in the simple case of F_2 matrices, I'm not sure it is easy to do better than brute force. But no, you don't need to write ...

14 days ago | 1

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Answered
Multi-Parameter function minimization with many many parameters
Let me try to help, a little at least. Why is fminsearch so bad at high dimensions? It is just not designed for that class of ...

16 days ago | 0

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Answered
Solve nlp equation system with variables spanning more than 40 powers of 10
Are the variables potentially going to vary by that many powers of 10? If so, then you would have the solver work in a log space...

19 days ago | 0

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Solve 𝑔 ′ ( 𝑥 ) = 0 for z in terms of N, a positive integer
First, it is not a cubic equation! Yes, you can replace sqrt(x) with a new variable z, thus a transformation. But doing so can ...

20 days ago | 0

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Answered
With k（0<k<0.5） and u（-1<u<1) as independent variables and x and y as dependent variables, how do you graph x and y, respectively, for a system of implicit function equations?
You have 2 parameters, k and u. If you were to fix those parameters, then you could solve for the solutions. There would be zero...

20 days ago | 0

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4 nonlinear equations by using Newton's method
Why are you writing Newton's method code to solve a problem? Never write code to do numerical methods work, if far better code i...

22 days ago | 0

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Is there a function in matlab to find temperature, density ecc.. values outside the standard atmposphere?
Um, not really, and certainly not easily or in any good way. Something like temperature can vary HUGELY outside the atmosphere,...

23 days ago | 0

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Issue regarding the result from symbolic computation
If you have this: a11 = 1/2; a12 = 1/2; a22 = 1/2; A = [a11 a12 0; a12 a22 0; 0 0 0] The eigenvectors were found as: [V,D] =...

24 days ago | 1

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Calculating Perpendicular Distance Between Detected Edge and Smoothing Function
If your curve is represented as a sequence of points (which is always possible) then you can use my distance2curve utility. It f...

24 days ago | 1

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Answered
How to draw the function relationship graph between two or three variables, where these variables are solved by a system of equations.
Not everything you might want to do has an answer. You ask to "draw" the relationship between variables. (For a fairly vaguely s...

25 days ago | 0

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How to create a smooth curve through data points?
Just coming back to this, as I did not see the data provided. (Sorry to @Gerard Feeney) But this is a difficult thing for multi...

26 days ago | 1

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Is it possible to select the parameters in the approximation equation using MATLAB?
Yes. Use the curve fitting toolbox. help fittype help fit Note that the estimates of these coefficients will be very uncertai...

27 days ago | 0

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HI, while doing a small calculation part, from my class note, in matlab, im getting the result as inf, can i know how to convert the inf value into realnumbers.
Of course D0 is undefined. You created the variable Do. There is a difference between the number 0 and the letter o. That Do is...

28 days ago | 0

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Need help with curve fitting
Um. NO. Yes, there is a language problem, and what you write is confusing. But you are not doing what you claim to want to do. W...

1 month ago | 1

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solving a nonlinear equation with complex numbers
syms R positive % positive implicitly implies real also syms X real syms Zo Z=(R*1i*X/(R+1i*X)); r=(Z-Zo)/(Z+Zo) r = simpli...

1 month ago | 0

Answered
Warning: Equation is badly conditioned. Remove repeated data points or try centering and scaling.
Do NOT use polynomials to model data like this In fact, do not use high order polynomials, pretty much ever. FR = load('FR.txt'...

1 month ago | 2

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Recursively reversing large vector efficiently
An old question with an already good answer. Regardless, it explicitly asks about recursive schemes to flip a vector. And I can ...

1 month ago | 0

Answered
Fixing biased random number generation
What is the goal of this? That is a good question, as asked by @埃博拉酱. You should recognize that the result of this operation w...

1 month ago | 0

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My end value "tc2" comes out as "NaN + NaNi", in the following script and I cant figure out why.
A NaN means you are doing some operation that has no meaningful result. For example, what is 0/0? 0/0 A mathematician will tel...

1 month ago | 0

Answered
Sparse matrix memory understanding
I think you missed a zero. By the way, 5% sparse is NOT at all what I would consider sparse, or for there to often be a gain. L...

1 month ago | 0

John D'Errico

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