The "2013" is the copyright date of the initial publication of the online edition of The Feynman Lectures on Physics, but that edition (as well as the printed edition) and the Feynman Lectures Website where it is published is far from static. I have been working on it continuously since 2006 (with occasional help from volunteers), during which time a lot of improvements have been made and much new material has been added (to the website - we are not adding new material to the printed edition, though we have made, and continue to make, corrections, as needed).
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