Curve fitting using a equation that involves a integral that isnt possible to solve analytically?

Question

simar 2011-7-8

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/11137-curve-fitting-using-a-equation-that-involves-a-integral-that-isnt-possible-to-solve-analytically

Hello there,

I'm trying from 2 days to curve fit some data that I have using this equation

f = @(x) ((x.^4) .* exp(x)) ./((exp(x)-1).^2);
gama*x + 9*R*((x/a)^3)*quad(f,0,a/x);

Here x is independent variable and a is unknown, gama is known. I tried the following procedure with the most success.

function C=myquad(a,T)
C = zeros(size(T));
gama = 20 * 1e-3;
R = 8.314;
f = @(x) ((x.^4) .* exp(x)) ./((exp(x)-1).^2);
for n = 1:length(T)
    C(n) = gama*T(n) + 9*R*((T(n)/a)^3)*quad(f,0,a/T(n));
end
>>fit(T_0,C_0,fittype('myquad(a,x)'));

It returned following error

??? NaN computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients.

Error in ==> fit at 443 errstr = handleerr( errid, errmsg, suppresserr );

No idea what to do. Please guide..

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Sean de Wolski 2011-7-8

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/11137-curve-fitting-using-a-equation-that-involves-a-integral-that-isnt-possible-to-solve-analytically#answer_15242

在 MATLAB Online 中打开

dbstop if error

then inspect the variables being fed into your function. A good place to start at least.

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

simar 2011-7-8

thanks a lot i spotted out ..

i found that when i change the integral limits from 0 to 0.1 or so .. it is able to calculate. But I'm not sure whether my method for fit is right or not. I'm not getting the correct curves.

请先登录，再进行评论。