Something that covers calculus of variations, Euler-Legrange equation, etc. I first covered this in Classical Mechanics but don't remember the textbook. The Fey

Something that covers calculus of variations, Euler-Legrange equation, etc. I first covered this in Classical Mechanics but don't remember the textbook. The Feynman Lectures of Physics [2] probably covers it, but I don't know for certain. Indeed it does: https://www.feynmanlectures.caltech.edu/II_19.html https://www.feynmanlectures.caltech.edu/II_19.html The original lecture recording is also available: https://www.feynmanlectures.caltech.edu/flptapes.html https://www.feynmanlectures.caltech.edu/flptapes.html