[is invalid?] cannot define function variables?

[zimmermann6]

It seems not possible to create a functional variable in SAGE. This gives strange things:

sage: var('f');
sage: f(x)
x

Ideally one should be able to do the following, to compute the formal derivative of f(g(x)):

sage: var('f,g');
sage: diff(f(g(x)), x)

(Currently this gives 1 due to the above strange simplification f(x) -> x.)

Component: calculus

Issue created by migration from https://trac.sagemath.org/ticket/1443

[mwhansen]

comment:1

sage: f = function('f')
sage: f(x)
f(x)
sage: g = function('g')
sage: f(g(x))
f(g(x))
sage: diff(f(g(x)),x)
diff(f(g(x)), x, 1)

[williamstein]

comment:2

Use function, as mentioned above:

sage: function('f, g')
(f, g)
sage: diff(f(g(x)), x)
diff(f(g(x)), x, 1)