# Van der Waals equation - Newton's method

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Blanca Mir Pou 2020 年 3 月 24 日
コメント済み: Blanca Mir Pou 2020 年 3 月 25 日
Hi
I'm stuck and I keep getting this error
Not enough input arguments.
Error in fun (line 4)
F(1) = log((3*x - 1)/(3*y - 1)) + 9/(4*T) * (1/x - 1/y) - 1/(3*x - 1) + 1/(3*y - 1);
Error in newton_ndim (line 6)
resd = [norm(feval(F,xk))];
Error in practica5 (line 16)
[XK,resd,n]=newton_ndim(@fun,X,tol,itmax);
Here is my main script
X = [1.2 ; 0.8];
Vk = [];
tol = 1e-16;
itmax = 100;
for T = 0.99:-0.01:0.85
[XK,resd,n]=newton_ndim(@fun,X,tol,itmax);
Vk=[Vk XK(:,end)];
X = XK(:,end);
end
The function I'm trying to solve is this one, obtained from Van der Waals equation
function F = fun (X,T)
x = X(1); y = X(2);
F(1) = log((3*x - 1)/(3*y - 1)) + 9/(4*T) * (1/x - 1/y) - 1/(3*x - 1) + 1/(3*y - 1);
F(2) = (8*T)/3 * (1/(3*x-1) - 1/(3*y-1)) - 1/x^2 + 1/y^2;
end
Do you know how to solve this?

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### 採用された回答

Are Mjaavatten 2020 年 3 月 25 日
In your code, newton_ndim does not know the value of T and so cannot call fun(X,T). You can probably fix this by defining an inline function (here called ffun) inside your loop:
for T = 0.99:-0.01:0.85
ffun = @(X) fun(X,T);
[XK,resd,n]=newton_ndim(ffun,X,tol,itmax);
Vk=[Vk XK(:,end)];
X = XK(:,end);
end
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Blanca Mir Pou 2020 年 3 月 25 日
Thank you, it works

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