Answer by benjaminfjones for I have an expression like uR(t) == 3*iL(0) + uC(0)/2 - 4how can I substitute for iL(0) and uC(0), if I'm given, that iL(0) = 0 and uC(0) = 0.uR, iL, uC are a functions of var t:t = var('t') uR = function('uR', t) iL = function('iL', t) uC = function('uC', t) thank you :)

The simplest way is just to use the `subs` (or `substitute`) method of your symbolic expression like so: sage: t = var('t') sage: uR = function('uR', t) sage: iL = function('iL', t) sage: uC = function('uC', t) sage: sage: SE = uR(t) == 3*iL(0) + uC(0)/2 - 4 sage: SE.subs(iL(0)==0) uR(t) == 1/2*uC(0) - 4 sage: SE.subs(uC(0)==0) uR(t) == 3*iL(0) - 4 sage: SE uR(t) == 3*iL(0) + 1/2*uC(0) - 4 You see from the last line that the object `SE` is not changed during the substitution, so you should assign the result of the substitution. Also, you can do both (or arbitrarily many) substitutions using a dictionary: sage: R = SE.subs({iL(0):0, uC(0):0}) sage: R uR(t) == -4