Generate a topic word for the given sentence. A topic word is usually the name of a person, place, animal, concept or object the sentence is focusing on.

the calculus of variations is a field of mathematical analysis.
calculus of variations

vichy is a "commune" in central france, in the auvergne-rhône-alpes region, along the allier river.
vichy

michigan avenue is a major north-south street in chicago.

michigan avenue