John D'Errico

Retired

Last seen: Today | Attivo dal 2006

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. Anyway. I won't answer questions. PERIOD. I don't do your homework. I won't do consulting by mail.

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3D Surface of revolution from a 2D perimeter
Ok. (I should probably write code to do this in general, for any arbitrary polygon.) Quad elements. I'll show how to do it for ...

circa 14 ore fa | 0

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Solving a set of 8 non-linear equations using symbolic toolbox results in empty sym 0 by 1
NO. The equation POSSIBLY has a solution. In theory, it MIGHT have a solution. But having a theoretical solution does not mean i...

circa 17 ore fa | 0

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Find Indefinite Integral from Array Values
Sorry. there is no need for them to be exactly the same. Cumtrapz is a simple variation of a fixed stepsize Euler integration,...

1 giorno fa | 1

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How can I find a multiplication coefficient between two signal?
Your need is to solve the problem signal2 = constant*signal1 Where the variable "constant" is unknown. This is easy to solv...

1 giorno fa | 0

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Equation solution is not found for all array element
For SOME of these sub-problems, no solution was found. I would suggest that when the result from vpasolve is empty, you could ...

1 giorno fa | 0

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How to reduce dimensionality of a rectangular matrix
Your matrix has size 640000 by 288. So there are only 288 pieces of distinct information in your data. You CANNOT use PCA to do ...

1 giorno fa | 0

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Symbolic acos( cos(theta) ) does not return theta.
Is it true, that acos(cos(theta)) ALWAYS returns theta? TRY AN EXAMPLE. acos(cos(10)) So it is not true. It works the other wa...

1 giorno fa | 0

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curve fitting of two equation to two data sets simultaneously
Are the values of x the same for the two vectors y1 and y2? You don't show any data, so it is difficult to help more, as there ...

2 giorni fa | 0

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Create a single variable to store multiple correlation coefficients
A = rand(300,7); B = rand(1,300); C = corrcoef([A,B']); C = C(8,1:7) The correlation coefficiens of each column of A, agai...

2 giorni fa | 1

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How do I remove every row that has a zero in it?
TSLA = rand(366,7); TSLA(TSLA(:,4) == 0,:) = []; One line. No loops.

3 giorni fa | 0

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Creating a mxn matrix that is all zeros except for the middle row and middle column which are 1s
Clearly a homework assignment. But now long since past, and with multiple answers, most of which are not in one line. However, s...

3 giorni fa | 0

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find the index number using bsxfun command
But why would you use bsxfun at all to do this? A = [12 13 15 1]; B = [1 2 12 15 10 13 11 14 3 4 5 6 7 8 9 16]; Use ismember ...

3 giorni fa | 0

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How can I rearrange equations to general form?
syms x y z eqn1=x+y+5==-z Nothing stops you from doing it yourself. For example, I might do: eqn1 = eqn1 + z Or, you can ev...

3 giorni fa | 0

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Inverse computation with a starting guess
It is often the case that someone THINKs they need a matrix inverse, yet they don't. And since you don't know if you can do this...

5 giorni fa | 0

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Vectorization approach to find matching elements in a matrix.
The part of your code that is a serious problem is where you build the arrays by growing them at each step. This is a really bad...

5 giorni fa | 0

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Please how can I calculate the values of this finite sum of numbers in fully sequential , I'm finding it difficult
Easy peasy. sumof4thpowers = @(N) nchoosek(N+1,5) + 11*nchoosek(N+2,5) + 11*nchoosek(N+3,5) + nchoosek(N+4,5); For example: s...

5 giorni fa | 0

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optimization in smart energy management system.I have used MILP algorithm but i got infeasible solution no point satisfy the constraints.how could i sort out these issues.
No solution means exactly what it sounds like. NO solution. And for this class of problem, if it tells you that NO solution exis...

5 giorni fa | 1

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How to find derivatives using the diff command
There are two versions of diff in MATLAB. One operates on numeric vectors and arrays. It simply subtracts each number from the o...

6 giorni fa | 0

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Sum two matrices shifted by a non integer number?
There is no magic, whereby you can do things that take computational time, and do them instantly. What you want to do is implic...

6 giorni fa | 0

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Falkner-Skan code
You write: tau = (mu/(2*delta_y))*(4*un,2-un,3) But what des that mean to you? Is the fragment (4*un,2-un,3) intended to be...

6 giorni fa | 0

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sqrtm vs chol to produce draws the multivariate normal distribution
Don't bother. sqrtm also can have a similar problem, but you won't like what it does either! For example: A = rand(3,2); A = A...

7 giorni fa | 1

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How to get tangent line from a point not on the curve
The curve is: syms x y x0 F(x) = 945.75*x.^0.1002; A line tangent to the curve at any spcific location x0 would be simple to ...

8 giorni fa | 1

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Solving systems of trig equations
My gut says no symbolic solution will exist. But that does not mean my gut is accurate here. First, I would throw pencil and pap...

8 giorni fa | 0

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Merging piecewise polynomial (pp) structures
Is there any method to "merge" two PP functions? Well, no. Should there be one? I've been using and writing splines tools for 40...

8 giorni fa | 0

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Solve of implicity equations
alpha = 0.14; A = 3300; n = 0.01 ; b_2019 = 69100; rho = 0.02; uno_alpha_A = ((1-alpha)*A); dos_rho = (2+rho); uno_n = (1+n);...

8 giorni fa | 0

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What am I doing wrong?
ex2d is a function. You call a function, even a function you wrote yourself, by passing in arguments. For example, when you use ...

8 giorni fa | 0

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Why can't the index variable of a for loop be stored in a structure or other array?
Surely this is not a big problem? for ind = 1:10 status.ind = ind; % do Stuff end

9 giorni fa | 0

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is there any way to get a 2nd order polynominal through 2 points(each end point)
You want to fit a 2nd degree polynomial through two points? Of course only a straight line would work there through two points, ...

9 giorni fa | 1

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Is there an option to overwrite the old release when installing a new one?
I'll admit, I typically have 3 to 6 releases, and usually at the high end of that scale. They are useful to me in case I might w...

9 giorni fa | 0

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Volume of retinal lesion
It seems clear. You have multiple slices. Compute the area of each slice. At that point, you could just sum the area of each sli...

9 giorni fa | 1

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