Today’s Tossup of the Day #3

by Gilbert Keith

Again, some of you might have seen this question before. Please comment on the quality of the question.

His theorem states that centroids of equilateral triangles erected on all sides of any given triangle form an equilateral triangle, though it is highly unlikely he discovered this fact. In a very famous painting, he is seen riding a horse and his name appears along with those of Hannibal and Charlemagne on the rocks of St. Bernard Pass. For ten points, identify this person who, on his return from Elba, ruled for hundred days, only to be defeated again at Waterloo.