Help on double integral

2012 3 5
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更新时间：2021 8 20
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I wrote a simple code for evaluating a double integral of a function of the following but it does n't work;
R = 0.0067; D = 1.1111;
f = inline('exp(-R*(y-x))*((((y+D)/(x+D)))^(R*(K+D)+1))*(1/((y+D)*y)))','x','y');
dblquad(f,0,100,100,inf)
Any with a solution? Thanks for your time

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Walter Roberson
Walter Roberson 2012-3-5
You have an extra ) at the end of the integral expression.

回答（1 个）

Walter Roberson
Walter Roberson 2012-3-5

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The above cannot be integrated, as it involves the undefined variable K .

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Natnael Hamda
Natnael Hamda 2012-3-5
sorry to mention it; K = 100;
Walter Roberson
Walter Roberson 2012-3-5
R = 0.0067; D = 1.1111; K = 100;
f = inline(sprintf('exp(-%.16g*(y-x))*((((y+%.16g)/(x+%.16g)))^(%.16g*(%.16g+%.16g)+1))*(1/((y+%.16g)*y))', R, D, D, R, K, D, D),'x','y');
Yes, the implication *is* that inline() never reads values of variables from the workspace: any variable whose value you do not pass as a parameter will cause an error.
I would suggest to you that using anonymous functions would be considerably cleaner than using inline(), if your MATLAB version supports anonymous functions.
R = 0.0067; D = 1.1111; K = 100;
dblquad( @(x,y) exp(-R*(y-x))*((((y+D)/(x+D)))^(R*(K+D)+1))*(1/((y+D)*y)), 0, 100, 100, inf)
Natnael Hamda
Natnael Hamda 2012-3-6
Thanks a lot; The anonymous functions creates the function in better way. But still there is an error when i try to evaluate the double integral. I included the error message:
??? Error using ==> mldivide
Matrix dimensions must agree.
Error in ==> @(x,y)exp(-R*(y-x))*((((y+D)/(x+D)))^(R*(K+D)+1))*(1/((y+D)*y))
Error in ==> quad at 77
y = f(x, varargin{:});
Error in ==> dblquad>innerintegral at 78
Q(i) = quadf(intfcn, xmin, xmax, tol, trace, y(i), varargin{:});
Error in ==> quad at 77
y = f(x, varargin{:});
Error in ==> dblquad at 54
Q = quadf(@innerintegral, ymin, ymax, tol, trace, intfcn, ...
Mike Hosea
Mike Hosea 2012-3-8
Your integral function must be able to accept arrays and return arrays. Don't use *, as this is matrix multiply. Instead use .*, elementwise multiplication. Similarly, don't use ^, rather .^. Don't use /, rather ./ . Basically, your integrand isn't working because it is being passed arrays, and it's trying to do matrix multiplications, matrix powers, and linear system solves when what you really just want is simple arithmetic, elementy-by-element.

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