Hi, I'd like to substitute the expression A into B, where B is of the form

B:=a*x+b*y+c*z+D(f)(p*x+q*y*r*z),

where a,b,c,p,q,r, are constants. The problem is: I only want the substitutions to be done outside of the unknown function f. So, if

A:=x=4*u,

Hi!

I have an expression that involves an unknown function, say g:

expr:=a*x[1]+b*x[2]+cx[3]+g(x[1],x[2],x[3]);

where a, b and c are the coefficients I'm actually interested in. These coefficients are not constants but usually expressions in some constant parameters. The function g is an expression involving derivatives of some more fundamental function (in my problem).

Example:

expr:=(q/r-sqrt(2))*x[1] + x[2] - (w*r+4)*x[3] + D(G)(x[1]) - D(G)(x[3]);