At http://mathworld.wolfram.com/FullWidthatHalfMaximum.html

there is a list of equations for the FWHM of different functions. For cosine, it’s

FWHM of cos(pi*x / 2*a) = (4/3)a

Given that cos goes from -1 to 1 and 0 is halfway between those, why wouldn’t the FWHM be from 0 to 0, ie 2*a?

Oh right, they’re doing FWHM from 0 to 1

Thanks for the question and the answer