What does the term "perturb" mean? I've been studying Calculus of Variations and I came a cross with the term "perturb" in my study material, but the term was not defined. The sentence where I read it from was: > "Rigid extremals are extremals that cannot be perturbed, and still satisfy the constraint." The material also uses the sentence: > "Perturbing around a curve"...? What does this mean? Thank you. P.S. Could you please post your answer as an answer and not as comment so that I can accept it ;)

According to my dictionary

**Peturb**

1. To disturb greatly in mind; disquiet

2. To throw in confusion; disorder

3. To cause (a moving body, celestial object, etc.) to deviate from a theoretically (orbital) motion.




I think we can ignore the first definition here but the other two are relevant . If you were to nudge a system slightly you are perturbing it and we may want to consider more stable if a small perturbation dies down and returns to a steady state quickly than a system which takes a long time to recover or one which never recovers.