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.

[EX Q]: the calculus of variations is a field of mathematical analysis.
[EX A]: calculus of variations

[EX Q]: vichy is a "commune" in central france, in the auvergne-rhone-alpes region, along the allier river.
[EX A]: vichy

[EX Q]: the world wide fund for nature (wwf) is an international environmental organization.
[EX A]:
world wide fund for nature