Error using eval with a differential equation
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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 Comment
Answers (2)
Walter Roberson
on 20 May 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
5 Comments
Walter Roberson
on 21 May 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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