Error using eval with a differential equation

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jose luis guillan suarez
评论： Walter Roberson 2018-5-21
when i do this i get this error:
>> t=0:0.01:10;
>> dsolve('D3x + 6*D2x + 11*Dx + 6*x=6*square(2*pi*t),D2x(0)=0,Dx(0)=0,x(0)=0')
ans =
exp(-t)*int(3*exp(x)*square(2*pi*x), x, 0, t, 'IgnoreSpecialCases', true, 'IgnoreAnalyticConstraints', true) + exp(-3*t)*int(3*exp(3*x)*square(2*pi*x), x, 0, t, 'IgnoreSpecialCases', true, 'IgnoreAnalyticConstraints', true) + exp(-2*t)*int(-6*exp(2*x)*square(2*pi*x), x, 0, t, 'IgnoreSpecialCases', true, 'IgnoreAnalyticConstraints', true)
>> y=eval(vectorize(ans))
Error using eval
Undefined function or variable 'x'.
what's happening? (i suppose is the square function because if i substitute it for sin(t) it works right.
  1 个评论
Stephen23
Stephen23 2018-5-19
编辑：Stephen23 2018-5-20
What do you hope to obtain by evaluating the output of dsolve? eval is not the right tool to use:
https://www.mathworks.com/matlabcentral/answers/400718-need-help-with-vector-and-eval#answer_320169
Is there a reason why you are using the very outdated char input to dsolve ?

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回答（2 个）

Walter Roberson
Walter Roberson 2018-5-20
Never eval() a symbolic expression. Symbolic expressions are not in the MATLAB language, just in something that is close to the MATLAB language.
But more immediately you need to
syms x
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Walter Roberson
Walter Roberson 2018-5-21
This is difficult to solve analytically, probably because the differentiation is not all that well defined right at the boundary conditions.
But you can get "good enough" this way:
syms t
Square(t) = piecewise(t - floor(t) < 1/2, 1, -1)
sol_plus = dsolve('D3x + 6*D2x + 11*Dx + 6*x=6*1,D2x(0)=0,Dx(0)=0,x(0)=0');
sol = Square(t) * sol_plus;
This works because solving for =6*-1 gives the negative of sol_plus.
Once you have the above you can plot with
T = 0:0.01:10;
solt = double( subs(sol, t, T) );
plot(T, solt);

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jose luis guillan suarez
i'm using R2017a

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