Function with a lower limit

edit

Could be how you're plotting the function. This works for me:

f(x) = 5 - 2*x
def g(x):
    return f(x) if f(x)>0 else 0
plot(g,x,-10,10)

With a slight modification to your first solution, this also works:

f(x) = 5 - 2*x
def g(x):
    return max(f(x),0)
plot(g,x,-10,10)

EDIT: Your plotting issues are a result of what Sage documentation calls "accidental evaluation". See item (4) on this page: http://doc.sagemath.org/html/en/tutor...

Concerning your second definition, as @paulmasson, i can not reproduce your problem.

Concerning you first definition, there is something tricky about symbolics to understand: when you write g(x) = max(f(x),0), the Python builtin max is used, which takes the max between two Python/Sage objects f(x) and 0, not pointwise! Since those two objects are not comparable, it considers that the first is to be returned, so max(f(x),0) is nothing but the object f(x). You can check by changing the order max(0,f(x)):

sage: g(x) = max(f(x),0)
sage: g
x |--> -2*x + 5
sage: g(x) = max(0,f(x))
sage: g
x |--> 0

So, you have to use is the "symbolic maximum", viewed as a mathematical function:

sage: g(x) = max_symbolic(f(x),0)
sage: g
x |--> max(-2*x + 5, 0)
sage: g.plot(-10,10)