How do I use a loop variable in the initial conditions in dsolve?

Question

Stephen Bosch 2013-6-27

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/80499-how-do-i-use-a-loop-variable-in-the-initial-conditions-in-dsolve

I am trying to build a loop which calls dsolve with different initial conditions in each iteration. To accomplish this, I built this loop:

y0 = -2:0.5:2;
y1 = -2:2:2;
syms y(t)
for i = y1
      for k = y0
       y(t) = dsolve(diff(y,2) + diff(y) - 2*y == t^2 - 4*t + 3,...
           ['y(0) == ',num2str(k)],['Dy(0) == ',num2str(i)])
       end
  end

It works through the first iteration, but it fails for the second one with this error:

Error using mupadengine/feval (line 157)
MuPAD error: Error: Invalid equation or initial condition. [ode::splitSys]
Error in dsolve>mupadDsolve (line 325)
T = feval(symengine,'symobj::dsolve',sys,x,options);
Error in dsolve (line 186)
sol = mupadDsolve(args, options);
Error in Blatt4_A5_MWE (line 9)
     y(t) = dsolve(diff(y,2) + diff(y) - 2*y == t^2 - 4*t + 3,...

The command works outside the loop for all the initial conditions I have tried. Inside the loop, the first iteration works, the next one always fails. It does not seem to matter which ones I choose.

What's going on?

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Answer 1

Stephen Bosch 2013-6-28

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/80499-how-do-i-use-a-loop-variable-in-the-initial-conditions-in-dsolve#answer_90242

在 MATLAB Online 中打开

You are assigning the first solution to the function y on the first iteration. When dsolve is executed again, y contains the first solution, and dsolve can't interpret that.

The answer is to use a different variable for the solution to the ODE, like so

y0 = -2:0.5:2;
y1 = -2:2:2;
syms y(t)
for q = y1
      for k = y0
       ysol(t) = dsolve(diff(y,2) + diff(y) - 2*y == t^2 - 4*t + 3,...
           ['y(0) == ',num2str(k)],['Dy(0) == ',num2str(q)])
       end
end