I really don't understand why the argument of the natural logarithm is greater than or equal to one. I don't know if this question sounds stupid, and I apologize in that case, but I am very new to funcions, so I am wondering if someone can explain me this passage here. Thanks a lot!
Crostul explains it well (in Italian), but for anyone not being able to decipher it, it is roughly:
If we have $\ln(t) \geq 0$, it follows (from exponentiating both sides) that $$e^{\ln(t)}=t\geq e^0=1$$