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# John D'Errico

### Retired

5,567 total contributions since 2006

DON'T contact me. I won't answer questions. PERIOD. I don't do your homework. I won't do consulting. A retired mathematician, numerical analyst, woodworker, bridge player.

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Array indices must be positive integers or logical values
The problem lies here: for i = 1:N za(i) = (z(i+1) - z(i-1))/(2*h); end When the loop starts out, what is i? i is 1, ri...

21 hours ago | 0

Help fitting data to an implicit equation
My thought would be the lazy solution. If your model is: ((1-y)^(1-b)/y)=exp(-kt) then log the model. That is, we know tha...

1 day ago | 0

How can I interpolate between my original endpoints with cscvn?
I think you do not understand splines. (Not uncommon.) Splines have the property that they can produce results that lie outsid...

1 day ago | 0

Function fitting on a set of data points
What did you do wrong? It looks like Jon may be correct, in that you did not use the newly estimated set of parameters from the ...

3 days ago | 0

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Question About Number Format in MATLAB Command Window
These are numbers from the symbolic toolbox. That toolbox does not use the format tools to display numbers (though perhaps argua...

3 days ago | 1

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How to Use "Ask a Question" Section Correctly?
It looks like this is a (surely quickly fixed) bug in the site itself. They recently made some changes, allowing users to use ...

3 days ago | 1

Precision (decimal digits) are very low and digits(32) does not help.
Of course, asking for 32 or more digits of precision is a bit on the side of the ridiculous, when the numbers going into the com...

4 days ago | 0

5 days ago | 0

array and inverse tan
I think you do not unerstand MATLAB. I'd start with taking the Onramp tutorial. atand of what? atand is a FUNCTION. You need to...

6 days ago | 0

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for loop help - Newton's method
I agree with much of what David has written (except for one thing). If you wish, feel free to accept his answer, as long as you ...

6 days ago | 0

Aerofoil graph interpolation/extrapolation
You lack sufficient information to construct a meaningful surface, since you have only 2 pieces of information in terms of Reyno...

6 days ago | 1

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how do i generate a torus?
WAY too much effort here to generate a torus. Start here: https://en.wikipedia.org/wiki/Torus [theta,phi] = meshgrid(linspace(...

6 days ago | 1

Using fminbnd on a Function with Multiple Outputs
Just evaluate your function one more time, after the optimization is done when you have the minimal location for x. This last ti...

7 days ago | 0

There is no analytical solution. syms x solve(cos(x)*cosh(x)+1==0) Warning: Unable to solve symbolically. Returning a numeric...

7 days ago | 0

Want to find exact answer for symbolic variable polynomial factorization
You say this is a polynomial. It cannot be a polynomial in c or n, since c and n live in the exponentis of p. Therefore you are ...

7 days ago | 0

Reduced Echelon form is wrong
A = [-2 2;-.4 .2]; [V,D] = eig(A) V = -0.974588432346814 -0.754384720454159 -0.224003097156669 ...

8 days ago | 0

Making complex number to trigonometric formula
Since this is clearly homework... Hint: what does angle do? (atan2 would also suffice, but then you would need to know how to e...

8 days ago | 0

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Find the volume under a 3D surface
Since this appears to be a gridded surface, then you can just use trapz, twice. Call it once on the x dimension, and once on the...

8 days ago | 0

How to contour this use matlab?
Are you asking how to write the code to take scattered data, and derive contours from pairs of points, using some variation of t...

9 days ago | 1

Generate random numbers in the second decimal point
Something like this is about the noise you want to see in that digit. If it is too large, then multiply by a little smaller numb...

10 days ago | 0

Error using betainc Z must be real and non-negative/polyfitn
You have 4 data points. You are trying to estimate a model with how many unknown coefficients? (20. Yes. 20.) I won't even get...

10 days ago | 0

How to find eigenvectors?
Why does V seem strange? A = [-2 0 2 2 -1 5 0 ...

11 days ago | 1

How to define a variable without using syms ?
fun = @(lambda) det(A - lambda*eye(size(A))); fun is a function of the unknown variable lambda. It is a function handle. It has...

12 days ago | 0

null(Matrix) returns an empty vector
That null returns an empty vector merely means your matrix is full rank. You cannot compute a non-empty null space of a full ran...

14 days ago | 1

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solution giving me imaginary answers.. Help me with this.
To be more clear, equations 5,6,8,9 merely establish the value of y1,y2,T,A. eqn5= y1-0.6==0; eqn6= y1+y2-1==0; eqn8= T==318....

15 days ago | 0

proble with differential equation
What did you do wrong? You wrote this: dsolve(diff(diff(f))+3*diff(f)==12*x-5), f(0) == 1, Df(0)== 0 And how is that different...

15 days ago | 0

Matlab 2020b won't open on Mac OS Big Sur
The simple solution of course to resolve any problems is to use R2021. It seems to run nicely on Big Sur. That is, given we are ...

15 days ago | 0

How to solve integral ==
You want to integrate exp(x), with limits from 1 to b?, and then to find b, such that the integral is 1? First, how do we perfo...

15 days ago | 0

Problems with Solving inequalities in matlab
That is not an error. It is a warning message, telling you there is a problem with what you have posed to solve. There is no ex...

16 days ago | 1