Unable to plot after using fsolve

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jayash
jayash on 7 May 2015
Answered: lei kai on 8 May 2015
Ut=0;
while Ut<20
cn = @(q1,q2,n0)((Ut.*n0)./(1-cos(2.*pi.*q1)-cos(2.*pi.*q2)));
y = @(q1,q2,n0)0.25.*((1+cn(q1,q2,n0)).^(-1/2)+(1+cn(q1,q2,n0)).^(1/2)-2);
a = -0.5;
b = 0.5;
v = @(n0) (quad2d(@(q1,q2) y(q1,q2,n0),a,b,a,b));
cv =@(n0) n0+(0.5*v(n0))-1;
x0 = 0.1;
n = fzero(cv,x0);
plot(Ut,n,'*')
xlabel('U/t')
ylabel('n0/n')
hold on
Ut = Ut + 1;
end
The plot should be similar looking like y=1/x but I am only getting a point. Anybody please help me out.

Answers (2)

Walter Roberson
Walter Roberson on 8 May 2015
You forgot to tell us that you also get some output, namely
Warning: Reached the maximum number of function evaluations (2000). The result fails the global error test.
> In quad2d at 244
In @(n0)(quad2d(@(q1,q2)y(q1,q2,n0),a,b,a,b))
In @(n0)n0+(0.5*v(n0))-1
In fzero at 286
Error using fzero (line 309)
Function value at starting guess must be finite and real.
That happens when Ut = 1 (the second iteration). The integration is failing. Are you sure the integral converges?

lei kai
lei kai on 8 May 2015
there is something wrong with your function cv,when Ut=1,n = fzero(cv,x0);will error

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