afni.nimh.nih.gov

Hello,
in 3dmerge help I see:
[in general, the
denominator in the exponent would be 0.36067 * FWHM * FWHM.
This is the only way to get a Gaussian blur combined with the
-1fmask option.]

I don't immediately see where 0.36067 comes from, please tell me. thanks,

George