Error using eval with a differential equation
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jose luis guillan suarez
el 19 de Mayo de 2018
Comentada: Walter Roberson
el 21 de Mayo de 2018
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.
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Respuestas (2)
Walter Roberson
el 20 de Mayo de 2018
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
el 21 de Mayo de 2018
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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