dsolve problem
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Hello, I would like to use a function f in a differential equation for example f=x but get a crazy result.
syms x y f
f=x
y=dsolve('D2x=-f','x(0)=1','Dx(0)=0')
How can I put into dsolve a function f defined outside of dsolve?
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Teja Muppirala
le 1 Juin 2012
One way would be to convert f to a string:
syms x y f
f=x
y=dsolve(['D2x=-' char(f)],'x(0)=1','Dx(0)=0')
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Walter Roberson
le 31 Mai 2012
You could try
syms x y f
f=x
eqn1 = subs(sym('D2x=-f'), 'f', f);
y = dsolve(eqn1, 'x(0)=1','Dx(0)=0')
3 commentaires
Walter Roberson
le 31 Mai 2012
Ah, I guess it doesn't analyze syms for occurrences of D. Ummm, I will have to make up a variable for x to be related to:
If I read the documentation right,
syms x(t) f(t) t
f = x(t); %or maybe f = x;
D2x = diff(x,2);
Dx = diff(x);
dsolve( D2x == -f, x(0) == 1, Dx(0) == 0 )
Anyhow, the root of the problem is that, as usual, when you have a quoted string like 'D2x=-f', MuPAD does not have access to the value of any variable such as f that was declared at the MATLAB level. subs() is the mechanism to "import" MATLAB-level values to the MuPAD level. It looks like dsolve() interprets symbolic variables differently than it interprets strings so the more obvious subs() does not work. char() around the subs() might perhaps have worked.
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