I agree that mathematically it's fairly certain that the aperture number is derived from it's relationship to the focal length. It's easy to make the assumption from that that f refers to focal length, but it's not necessarily so, and people have been arguing about it as long as I can remember. I did research this in depth yonks ago, and there are other theories for why the f was appended to the number, one being that it denoted 'focus' for the varying depth of field at different apertures, another being that it was derived from I think a latin word meaning literally 'window'.
Interesting nonetheless.
