Hi all,
It seems none of the current substitute methods for symbolic
expressions works for the argument of the derivative operator.
In computing functional derivative, I need to vary
a functional. For example, in sage-3.4 I can do as follows
-------
sage: f(x) = function('f',x)
sage: df(x) = function('df',x)
sage: g = f(x).diff(x)
sage: g
diff(f(x), x, 1)
sage: g.subs_expr(f(x)==f(x)+df(x))
diff(f(x) + df(x), x, 1)
-------
In new symbolics, if I do the same I get
--------
sage: g
D[0](f)(x)
sage: g.subs_expr(f(x)==f(x)+df(x))
D[0](f)(x)
---------
Is there a way to do this in new symbolics? It seems
to be a road block in implementing calculus of variations
in new symbolics.
Thanks,
Golam
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