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# John D'Errico

### Retired

4,882 total contributions since 2006

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

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Portfolio Optimization in support of Monte Carlo method.
You cannot use a tool like fmincon to optimize a stochastic function. That is, any function where repeated calls to the function...

2 hours ago | 0

inverting variables with two matrices
Too late to be of any value to the OP, but others may have the same question... In general, if you have LINEAR surfaces, thus p...

2 hours ago | 0

Lower Triangle of Matrix
No. You cannot tell triu to fill with ones instead of zeros. That does not mean what you are asking is impossible, just that you...

12 hours ago | 0

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How can I generate random natural numbers between min and max range?
help randi

1 day ago | 0

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'solve' function always returns zero for the case it should not be zero!!!
Surely you cannot claim that 0 is not also a solution. The solution you know of may be the solution you want, but that does not ...

1 day ago | 0

Cannot solve symbolic equation annalytically
"Does anybody has an idea why the function (which is basically only a difference of sums of polynomials with some logarythmic fu...

2 days ago | 0

gaussian elim with rref(M) over Gf(2)
Far too late for this question, but in case others want help... You seem to have asked two questions. First, how to create a sy...

3 days ago | 0

Volume of overlapping ellipsoids
The simple solution is just to fill a 3-d array with voxels that are inside each ellipsoid. So, if a voxel is already filled, th...

4 days ago | 0

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generate 0 and 1
rand() < 0.6 If you want a random binary sequence of length n, then this is sufficient: rand(1,n) < 0.6

5 days ago | 0

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interrupt a function when it takes too long
This is not what parfor is designed to solve, nor even what you could reasonably solve with parfor. Since you are calling the f...

5 days ago | 0

To create a code that differentiate any function
Well, you write a complete version of a set of symbolic tools, that can parse any function provided, and know the rules for diff...

6 days ago | 0

How can i extrapolate data?
Extrapolation is a dangerous thing to do. Best left to someone with sufficient expertise that they know when they are doing some...

6 days ago | 1

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How to Get Answers in Non-PolyGamma Form?
(I'm sorry that my answer is coming 9 years too late to be of any significant value to you personally. But someone else might fi...

6 days ago | 0

Making contours in polar plot
You could at least spell my name right, since you had it right in front of you. Regardless, it turns out that you changed your ...

7 days ago | 0

How to fit custom polynomial equation?
You could download polyfitn from the file exchange. But if you already know how to use the curve fitting toolbox, then just spe...

7 days ago | 0

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How to efficiently plot functions?
Wyhy not try it? I don't have t or y, so I'll make them up. t = (1:10)'; y = rand(10,12); plot(t,y) Actually, I don't need...

8 days ago | 1

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How can I creat a lower triangular with one vektor avoiding for loops?
Will it always be a matrix of that form? Or will you next ask how to fill the lower triangle with a fully general set of numbers...

8 days ago | 1

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How to use for loop and if else condition for multiple conditions
First, you don'r need a for loop that runs over system, sine you re just assigning it one value! That is, this: for system = 1 ...

8 days ago | 0

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Distance between a point and the intersection of two spheres
This is confusing, possibly because of how you are trying to solve it. I think you are working too hard. The intersection betwe...

9 days ago | 0

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How to plot polynomial function correctly?
Hmm. Why does this feel like deja vu? (Answer: I just answered almost the same question for someone else, not 5 minutes ago.) d...

9 days ago | 1

Plotting a graph with a confusing equation
How to lie with statistics. ;-) (Yes, that is the name of a little book I read long ago.) My point is, not that you or anyone i...

9 days ago | 1

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Matrices with different numbers of equations
You know the size of the matrix. You already know you need to use zeros to initialize it. A = zeros(2*N,2*N); b = zeros(2*N,1)...

9 days ago | 1

plotting a natural cubic spline
Are you allowed to use existing code? So if you have the curve fitting toolbox, you can just use csape. x = 0:4; y = [1 3 3 4 ...

9 days ago | 1

Find the vector sum combination that gives the minimum vector length
First, NEVER store data in multiple variables like this. Instead, LEARN TO USE ARRAYS. There is no reason why you cannot store a...

10 days ago | 2

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Unrecognized function or variable 'carsmall'
This is not a question of you updating your version. I can see what you did, but not what you were hoping to do. Suppose I try t...

10 days ago | 1

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Is the Trapz feature always accurate?
trapz is not a "feature". It is a piece of software that embodies the trapezoidal rule, something that students may be exposed t...

10 days ago | 1

Why do I get this error in my Taylor Expansion?
It might seem a subtle point. But what is the shape of sqrt(sin(x)), where x is near zero? (I'll just disregard the +2, since th...

10 days ago | 1

What is the reason that a Hessian should be positive definite? And what is the relation between an ill-conditioned Hessian and convergence?
(This is not really a MATLAB question, but you are asking a question about the behavior of MTLAB solvers. Therefore close enoug...

11 days ago | 1

Why can't I write this row vector?
Very often, people who are just starting to use MATLAB, think that if you don't know the value of something, then it must be sym...

11 days ago | 2

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