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# John D'Errico

### Retired

Last seen: Today Attivo dal 2006

Followers: 1   Following: 0

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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Risposto
Accessing a vector form a matrix
As an example, given vectors x1,x2,x3. I'll make some up. x1 = randi(10,1,5) x2 = randi(10,1,5) x3 = randi(10,1,5) Combine t...

circa 21 ore fa | 0

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Find the variable inside the LCM argument
You have the expression: B=lcm(nrt,Nr*Nc) where the variables B and nrt are known. The unknown variable is the product nr*...

circa 22 ore fa | 1

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volume calculation from the DelaunayTri(x,y,z) function
Its not THAT hard. You compute the volume of each simplex in the tessellation. Sum the absolute values up. (Since those simplexe...

1 giorno fa | 1

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is there any reason for this?
Is there any reason? Well, does there absolutely need to be a "reason"? These are a set of choices, made over a period of multip...

1 giorno fa | 1

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find a zero of a two-variable function
A two variable function is a surface. (@Sam Chak said this, but did not take the idea to the point of completion.) But if a sur...

3 giorni fa | 0

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Extracting values from optimization variable and setting constraints
Are you saying you want to constraint the sum of absolute values? That is, in context of a LINEAR programming problem? This is ...

4 giorni fa | 0

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I am trying to do multi step math and keep returning the wrong answer
Of course it is wrong. Why did you write 72/13? Why 25/3? I think you don't understand that 8(1/3) is NOT the same thing as ...

4 giorni fa | 0

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Calling GA() with 10 generations 10x vs GA() with 100 generations.
Is there any difference? Of course! Make it more extreme yet. Suppose you were to stop after 1 iteration. ONLY 1. But do it 100 ...

6 giorni fa | 0

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Check the result if it is divisible or not
Do you understand that you are using DOUBLE precision arithmetic to compute that mod? Do you understand that 7^2026 is a number ...

6 giorni fa | 1

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Treating ties in a Gaussian Elimination process
Um, max does exactly that. Where is there a problem? x = [1 2 3 4 3 4 1]; [xmax,ind] = max(x)

7 giorni fa | 0

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Matlab editor settings: notify undefined variables
For one example of a scenario where a variable appears to be undefined, yet is properly used, consider the case of a nested func...

7 giorni fa | 0

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Trying to determine roots of this polynomial (Det)
k = [1750 -750 0; -750 1250 -500; 0 -500 500]; m = [75 0 0; 0 75 0; 0 0 50]; D=inv(m)*k; syms x I = [x,0,0;0,x,0;0,0,x]; La...

7 giorni fa | 1

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Area using trapezoidal rule
Um, this is WRONG. Basic integral calculus does NOT say a result should never be negative. You are possibly misunderstanding wha...

9 giorni fa | 1

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Incorrect Coefficients from Symbolic Polynomial Equation Solving
NO. You think there is a unique solution. But that is wrong. syms a1 a2 a3 a4 x; % Define symbolic variables % Define the equ...

10 giorni fa | 0

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How do I extract one point from a piecewise function?
You can define a piecewise function as a symbolic function. For example... syms t M_xy(t) = piecewise(t < 0, 0, t > 0, t); % N...

10 giorni fa | 0

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Why isn't my MATLAB plot not showing up?
A STRONG suggestion is when you do something, to look at the results of your code. Don't just give up when you see nothing in th...

10 giorni fa | 0

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Estimating SQRT of a non-negative number the long way
If you generate an error message, MATLAB stops execution. An error is a failure, and nothing gets past that. However, a warning ...

11 giorni fa | 0

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Finding the index k of square nilpotent matrix A
A nilpotent matrix is a square matrix A that has the property that for SOME integer power k, we have A^k is entirely zero. The s...

11 giorni fa | 1

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Simultaneous Constant Optimisation for Curve Fitting
Do NOT manually search through a 15 dimensional space for a solution. You are just wasting your time, and this is why tools are ...

11 giorni fa | 1

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How do I graph the negative portion of a square root plot?
Since in reality, you have what is called an implicit function, we want to use fimplicit. I might do it like this: fun = @(x,y)...

13 giorni fa | 1

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How to invert a square matrix using Gaussian elimination
I assume you are forced to use your own code for gaussian elimination here. A bad idea if you are not. NEVER write such code you...

13 giorni fa | 0

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Lognormal distribution parameters mu and sigma
In MATLAB, the tools that use a lognormal distribution have you provide mu and sigma in terms of the underlying normal distribut...

14 giorni fa | 0

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lsqlin unexpected results !!!
Unfortunately, you would have been better off using a direct least squares. That would allow us to see how well the calibration ...

14 giorni fa | 1

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Trying to find pi using when loops to a tolerance of 10^-4.
Many, many things wrong in your code. I'm amazed nobody else as pointed most of them out. It was a start in the correct directio...

15 giorni fa | 2

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how to use a variable in finite field
g = gf(3, 8) whos g g is a gf object. But g is not compatible for multiplication by a symbolic parameter. These are two indepe...

17 giorni fa | 0

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code for check wether the given differential function is function g(x) or g(x,y) in matlab
Easy. For example... syms x y expr1 = 2*x; expr2 = x + x*y; Now use symvar. symvar(expr1) numel(symvar(expr1)) symvar(exp...

17 giorni fa | 1

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can any one help me to find simple empirical model (equation) with a little parameters for this data
As an alternate model, one that employs the assumption that the curve passes through the point (0,1), can we find a different fi...

17 giorni fa | 0

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using global variables to avoid passing by value
Sorry. I'll call it a bad idea, generated because you erroneously think you are doing something good. MATLAB does not actually ...

17 giorni fa | 1

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is there a way to make the 0 x and y axis bold?
It seems simple enough to me. If you want to have ONLY the x and y axes emboldened... % arbitrary example plot, and there is no...

17 giorni fa | 0

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can any one help me to find simple empirical model (equation) with a little parameters for this data
Why not try it? This seems adequate: x=[31.96 30.25 28.27 26.84 25.4 24 23.27 21.89 20.85 18.6 17.02 16 15.81 13.6 12 11.19 9....

17 giorni fa | 0